Windows's SRW-lock should be replaced by a custom recursive untied-to-thread lock.

[erthink]

Windows is unable to extend or shrink mapped region without closing/unmap it.
Theresofre, on Windows a SRW-lock is used to protect threads with a read transaction from accessing a temporary unmapped region during it shrink or expand.
However, with MDBX_NOTLS option transactions could be used from other threads and the same thread could start more than one read transaction.
Currently, to prevent this, libmdbx does not shrink a DB file and does not increase the mapping size when using the MDBX_NOTLS option.

So SRW-lock should be replaced by a custom recursive untied-to-thread lock.

[erthink]

In continue to #203, pre-requirement for #200.