CDRIVER-2376 fix performance loss

Starting in commit #9f9934dc, the driver had incorrectly called "ping"
before nearly every command when in single-threaded mode. This was due
to the mongoc_topology_scanner_node_t.last_update field not being
updated, so the driver thought each server's socket had been idle and
needed to be checked with "ping", even when the socket had been used
recently.

In this commit, update last_used after every command so we don't call
"ping" unnecessarily.