Drop the mutex on the multiplexor before entering the xRead VFS call.
Fix temporary directory separator handling for Cygwin.
Merge change to drop the mutex on the multiplexor before entering the xRead
VFS call, in order to enhance parallelizability.
Performance improvements:
Avoid unnecessary seeks when doing a single-row UPDATE on a WITHOUT ROWID
table.
Performance improvement: Avoid unnecessary seeks on REPLACE INTO for a
WITHOUT ROWID table.
Merge the Cygwin directory separator fix.  Also fix a C++-ism in the
multiplexor code so that it will compile on MSVC.
Fix harmless compiler warnings.