Limit use of path::identifier_t
Use std::mutex instead of OSSpinLock
Fix path/inode lookup when opening documents

The strategy is to prefer an existing document with same path and if there is none, check if there is a document with the same inode. The latter is so that links or files that have been moved, do not create duplicate documents.

The old code used a single map for the lookup, using path::identifier_t as key (wrapping path and/or inode), though since the map is ordered, the code would occasionally fail, as the keys cannot be correctly ordered (so effectively this reworked code is a fix for an incorrect path::identifier_t::operator<).

Fixes #1052.
Remove path::identifier_t