MEDIUM: ssl: manage shared cache by blocks for huge sessions.

MEDIUM: ssl: manage shared cache by blocks for huge sessions.

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.