add option to limit WiredTiger cursor cache size

This change introduces a new setParameter wiredTigerCursorCacheSize with a default value of 10,000.

When this is set to 0, caching of WiredTiger cursors is disabled. There is an expected and slight performance degradation from the loss of the cache, but resource usage will be lower because we are not keeping as many cursors open at once. This parameter can be safely set at runtime without requiring a restart of the server.

This will speed up dropDatabase, collection drop, and other operations.