SSL: reduced logging of session cache failures (ticket #621).

SSL: reduced logging of session cache failures (ticket #621).

Session cache allocations might fail as long as the new session is different
in size from the one least recently used (and freed when the first allocation
fails).  In particular, it might not be possible to allocate space for
sessions with client certificates, since they are noticeably bigger than
normal sessions.

To ensure such allocation failures won't clutter logs, logging level changed
to "warn", and logging is now limited to at most one warning per second.