Add table cache capacity and usage properties

Add table cache usage and table cache usage values to the property table. is-write-stopped also added, though unclear if useful.

Fix thread join crash when thread unused

New option to disable worker thread lead to shutdown crash. Calling thread::join() on unused thread object bad. Appears protective call to thread::joinable() is safe and fixes problem.

Add flag to disable worker thread at runtime

1. StatsTable constructor now takes a flag for whether or not to start the worker thread.
2. Memory leak / bug found in AddEvent() routine.
3. Looks like rocksdb::Statistics has a memory leak on destruction. Did not resolve, yet.

Snmp agent updated for rocksdb statistics tables

rocks_snmp_1.1

add subset of variables available via GetAggregatedIntProperty