Cherry pick changes from IndexStoreDB's copy of LMDB

[matthewbastien]

IndexStoreDB enables thread sanitization in its builds and made several changes to its copy of LMDB in order to build:

1. Change uses of volatile to _Atomic
2. Enable POSIX mutexes on non-Windows platforms
3. Disable robust mutexes since they only apply to multi-process access of databases

IndexStoreDB also sets MDB_MAXKEYSIZE to 0 in order to tell LMDB to use the max computed key size.

[QuietMisdreavus]

This looks great, thanks! I'm glad swift-lmdb is getting picked up by more projects; it's otherwise a forgotten component of Swift-DocC. 😅