Rate limit renegotiations
Currently tor allows any number of renegotiations because:
a) tor_tls_block_renegotiation() won't do it for rfc5746 renegotiations. b) The renegotiation callback which calls tor_tls_block_renegotiation() is only called on the first Application Data packet instead of when the renegotiation takes place. This is because the SSL_read() return value is not treated correctly.