Sessions using client certs are huge (more than 1 kB) and do not fit
in session cache, or require a huge cache.
In this new implementation sshcachesize set a number of available blocks
instead a number of available sessions.
Each block is large enough (128 bytes) to store a simple session (without
client certs).
Huge sessions will take multiple blocks depending on client certificate size.
Note: some unused code for session sync with remote peers was temporarily
removed.
af9619da3e