tls: mitigate session renegotiation attacks

The TLS protocol allows (and sometimes requires) clients to renegotiate the
session. However, renegotiation requires a disproportional amount of server-side
resources, particularly CPU time, which makes it a potential vector for
denial-of-service attacks.

To mitigate this issue, we keep track of and limit the number of renegotiation
requests over time, emitting an error if the threshold is exceeded.