Define UNIV_WORD_SIZE as a simple alias to SIZEOF_SIZE_T.
In MariaDB 10.0 and 10.1, it was incorrectly defined as 4 on
64-bit Windows.

MONITOR_OS_PENDING_READS, MONITOR_OS_PENDING_WRITES: Enable by default.

os_n_pending_reads, os_n_pending_writes: Remove.
Use the monitor counters instead.