Introduce SSH session limit

Introduced property plugin.ssh.session.max (default value is 250) to limit to the number of concurrent SSH sessions which can be opened in the system.

This limit applies to each node in a Data Center setup, so each node will be able to serve plugin.ssh.session.max sessions concurrently.

Each SSH session has a memory footprint which depends on the commands which are being executed (among other things). In certain situations, a large enough number of sessions, could eventually exhaust the heap space and produce an OutOfMemoryError. This limit adds another protection so that on load spikes the system's performance and reliability will not suffer.

Users who encounter this limit should see a message similar to this:

Received disconnect from ::1 port 7999:12: Too many concurrent connections (250) - max. allowed: 250
Disconnected from ::1 port 7999
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.