Even before this change SockJS sessions always cancelled the heartbeat
task first prior to sending messages. However when the heartbeat task
is already in progress, cancellation of it is not enough and we must
wait until the heartbeat is sent.

This commit adds a heartbeat write lock which is obtained and held
during the sending of a heartbeat. Now when sessions send a message
they still cancel the heartbeat task but if that fails they also wait
for the heartbeat write lock.

Issue: SPR-14356