SERVER-15535 Retire heartbeat.js, which tests functionality no longer supported.

The test jstests/replsets/heartbeat.js confirmed that recipients of heartbeat
requests would consider the senders to be "up", even if they had not recently
sent a successful heartbeat message to the sender.  This aimed to reduce
flapping during short asymmetric network partitions, which functionality is now
achieved by retrying failed heartbeats up to two times before marking a node as
down.
SERVER-15535 Cancel outstanding network requests in replication during shutdown.