Use Generic Name for SimplePool Connection Threads (1.6.x)

[miklish]

PR: #2090

Currently SimplePool connection threads are named either "Callback-HTTP11" or "Callback-HTTP11" depending on the type of connection.

However, since this threadpool is only named ONCE, then the name of all SimplePool connection threads is based on the first type of connection that SimplePool was asked to make (HTTP/1.1 or HTTP/2).

This can cause misleading messages in the logs, since if subsequent calls to the pool create different connection types, then the thread name and connection type won't match.

E.g.: If an HTTP/1.1 connection is the first connection the pool makes, then any subsequent HTTP/2 connection threads will have the name "Callback-HTTP11".