Rework mongoose_mam_id as a persistent_term with an atomic

```elixir
  def next_id() do
    Benchee.run(
      %{
        "native code" => fn list ->
          Enum.map(list, fn t -> :mongoose_mam_id.next_unique(t) end)
        end,
        "pure erlang" => fn list ->
          Enum.map(list, fn t -> :mongoose_mam_id_2.next_unique(t) end)
        end,
      },
      inputs: %{
        "1M monotonic" => monotonic_list(100_000_000)
      },
      formatters:
      [
        {Benchee.Formatters.Console, extended_statistics: true}
      ],
      parallel: 1,
      time: 5,
      warmup: 0
    )
  end

  defp monotonic_list(size) do
    fun = fn(_, [head | _] = acc) ->
      next = head + :rand.uniform(20) - 1
      [next | acc]
    end
    :lists.reverse(:lists.foldl(fun, [:rand.uniform(20)], :lists.seq(1, size)))
  end
```

returns

```
Operating System: macOS
CPU Information: Apple M3 Pro
Number of Available Cores: 12
Available memory: 36 GB
Elixir 1.17.3
Erlang 27.1.2
JIT enabled: true

Benchmark suite executing with the following configuration:
warmup: 0 ns
time: 5 s
memory time: 0 ns
reduction time: 0 ns
parallel: 1
inputs: 1M monotonic
Estimated total run time: 10 s

Benchmarking native code with input 1M monotonic ...
Benchmarking pure erlang with input 1M monotonic ...
Calculating statistics...
Formatting results...

Name                  ips        average  deviation         median         99th %
pure erlang          0.31         3.23 s    ±10.49%         3.23 s         3.47 s
native code          0.28         3.54 s     ±6.26%         3.54 s         3.70 s

Comparison:
pure erlang          0.31
native code          0.28 - 1.10x slower +0.32 s

Extended statistics:

Name                minimum        maximum    sample size                     mode
pure erlang          2.99 s         3.47 s              2                     None
native code          3.39 s         3.70 s              2                     None
```

rebar.config

@@ -37,11 +37,6 @@
     {httpd_util, integer_to_hexlist, 1}
 ]}.
 
-{port_specs,
- [
-  {".*", "priv/lib/mongoose_mam_id.so", ["c_src/mongoose_mam_id.cpp"], [{env, [{"CXXFLAGS", "$CXXFLAGS -O3 -std=c++11"}]}]}
- ]}.
-
 {require_min_otp_vsn, "21"}.
 
 %% We agreed to use https:// for deps because of possible firewall issues.

src/ejabberd.erl

@@ -25,9 +25,7 @@
 
 -module(ejabberd).
 -author('[email protected]').
--export([get_pid_file/0,
-         get_status_file/0,
-         get_so_path/0]).
+-export([get_pid_file/0, get_status_file/0]).
 
 -type lang() :: binary().
 
@@ -41,20 +39,6 @@
 
 -export_type([lang/0, xml_stream_item/0]).
 
--spec get_so_path() -> binary() | string().
-get_so_path() ->
-    case os:getenv("EJABBERD_SO_PATH") of
-        false ->
-            case code:priv_dir(mongooseim) of
-                {error, _} ->
-                    ".";
-                Path ->
-                    filename:join([Path, "lib"])
-            end;
-        Path ->
-            Path
-    end.
-
 -spec get_pid_file() -> 'false' | nonempty_string().
 get_pid_file() ->
     case os:getenv("EJABBERD_PID_PATH") of

src/mam/mongoose_mam_id.erl

@@ -20,9 +20,28 @@
 -export([next_unique/1]).
 -on_load(load/0).
 
+-spec load() -> ok.
 load() ->
-    Path = filename:join(ejabberd:get_so_path(), ?MODULE_STRING),
-    erlang:load_nif(Path, 0).
+    case persistent_term:get(?MODULE, undefined) of
+        undefined ->
+            Atomic = atomics:new(1, [{signed, false}]),
+            persistent_term:put(?MODULE, Atomic);
+        _ ->
+            ok
+    end.
 
-next_unique(_Candidate) ->
-    erlang:nif_error(not_loaded).
+-spec next_unique(integer()) -> integer().
+next_unique(Candidate) when is_integer(Candidate), 0 < Candidate ->
+    Atomic = persistent_term:get(?MODULE),
+    next_unique(Candidate, Candidate - 1, Atomic).
+
+-spec next_unique(integer(), integer(), atomics:atomics_ref()) -> integer().
+next_unique(Candidate, Current, Atomic) ->
+    case atomics:compare_exchange(Atomic, 1, Current, Candidate) of
+        Int when is_integer(Int), Candidate =< Int ->
+            next_unique(Int + 1, Int, Atomic);
+        Int when is_integer(Int) ->
+            next_unique(Candidate, Int, Atomic);
+        ok ->
+            Candidate
+    end.