New "leaky bucket" algorithm for empty message denial-of-service countermeasure. Previously, the count of empty messages was continually being incremented by the MatrixSSL library regardless of any interleaving valid messages. This could potentially cause the connection to be closed if a peer was sending many blank SSL messages. The count will now decrement on valid messages. This change is most relevant to use-cases that involve an OpenSSL client communicating with a MatrixSSL server, as these clients tend to periodically send a blank record.
