If a first packet is sent and its answer received in time, then a second
packet is sent after more than PING_DELAY seconds, a ping will be emitted
right after that second packet, because _ping_time was still set.
This change resets it when there is no pending requests.

Also, as required to have a consistent view of _handlers, check that lock
is properly held in the MT variant of the class.

git-svn-id: https://svn.erp5.org/repos/neo/trunk@2033 71dcc9de-d417-0410-9af5-da40c76e7ee4