Check pointer before dereferencing SERVER-1929
Track heartbeats received for health SERVER-1929

Conflicts:
	src/mongo/db/repl/health.cpp
	src/mongo/db/repl/rs.h
Add retry to heartbeat SERVER-1929

Conflicts:
	src/mongo/db/repl/heartbeat.cpp
Add heartbeat timeout setting SERVER-1929

Conflicts:
	src/mongo/db/repl/rs_config.cpp
	src/mongo/db/repl/rs_config.h
SERVER-1929 Set return value on heartbeat retry
SERVER-1929 Remove unused heartbeat options from stepdown logic

Fixed test because stepdown is so much faster that the connection is dead
by the time ismaster is called.
SERVER-1929 Fix test to handle a stepdown
SERVER-1929 Prevent outgoing heartbeat ports from closing on stepdown
SERVER-1929 Replace literal with a constant

Conflicts:
	src/mongo/db/repl/rs.cpp
SERVER-1929 Fix test race condition

Conflicts:
	jstests/replsets/stepdown.js