Share per-table cursors for get/put/delete operations.

This change adds an ets table mapping table name to an open shared cursor for operations (get/put/delete) on that table to avoid open/use/close cycle of calling the session_{get,put,delete} operations.  This should save us some time going in and out of the WT library.

One open question is if the cursor handles could potentially become invalid for reuse due to some error condition in the library.