PingStats::hit() should not set _numFailuresSinceLastStart to integer max

PingStats, defined in ToplogyCoordinatorImpl, is a data structure used to track statistics about replication heartbeat messages. It has a variable called _numFailuresSinceLastStart, which keeps track of failed heartbeat attempts. When a good heartbeat response comes back, this variable is set to std::numeric_limits<int>::max(), presumably as an easy way for this value to always be larger than others in certain comparisons. In the case that we actually get back a good heartbeat response (PingStats::hit) and then a failed heartbeat from the same node (PingStats::miss), we will actually overflow this _numFailuresSinceLastStart variable. We should not use the integer max as a way to determine that we received a good heartbeat response. Creating a separate field in PingStats to explicitly track this would probably be a simple approach.