Add the session-statistics to the statistics cursor, __wt_cursor_stat.

Implement the earlier placed stub, __curstat_session_init to copy a snapshot of the session-statistics structure from WT_SESSION_IMPL into __curstat_session_init.

Scope includes updating all the curstat methods as required to support session statistics.

To add a test:

1. Open a session statistics cursor on a session for which session statistics collection is disabled.
2. Open a session statistics cursor on a session for which session statistics collection is enabled.
3. Open a session statistics cursor on a session for which session statistics collection is enabled and fetch statistics with out performing any other operation on the session.