magic-folder-api should load config read-only

[meejah]

There are some occasional spurious integration test failure on Windows and MacOS to do with "database being locked".

My current theory is that this is due to running magic-folder-api subcommands when the "main" process is doing database-stuff (although I'm unsure why they don't just fail 100% of the time, unless sqlite3 only actually locks the database when there's a write transaction?)

In any case, it would be better practice to have magic-folder-api only open in read-only mode since it doesn't need write-access and -- ideally -- that would also fix these occasional problems.

(It would also be possible to fix by just doing HTTP transaction instead of running magic-folder-api commands, but the latter is what most downstream CLI users would usually do so it's a better "integration test" IMO if the integration tests do that too...)

[meejah]

See also #339

[meejah]

Investigation required: how does locking work with sqlite, and is it platform-dependent?