Remove internal Lock/Reader/Writer structs from System.Composition.Convention

[Clockwork-Muse]

Per #2995, remove ReaderLock stuct (and the related WriterLock/Lock structs) in preference for addressing the underlying lock (ReaderWriterLockSlim) directly.

Fixes #2995

@dsplaisted , @stephentoub

[stephentoub]

Thanks, @Clockwork-Muse.

I'm not opposed to the notion of structs similar to these, but a) the interlockeds they're using are pure overhead if the structs are used correctly and are dangerously misleading if they're not (since two copies of the struct would not communicate), b) they're only used in a few places in the whole codebase (the write one is used only once), and c) this special Lock type doesn't seem to provide any value and is itself just overhead (maybe the intention at one point was to be able to plug in a different lock?)

In any event, LGTM.

[Clockwork-Muse]

@stephentoub - I was debating the intent of c a little, yeah, but wasn't coming up with much good other than either having a new regular ReaderWriterLock, or something that would likely require changing that abstraction... /shrugs/ maybe it's from before the RWSlim? Seems unlikely, though.