Implement multithread

[timweri]

Rustica Agent now spawns a thread to handle each request. Mutexes are used to ensure thread-safe concurrent accesses to some fields.

Tests done:

• ssh -T [email protected]
• 8 concurrent subprocesses calling ssh -T [email protected]
• 4 concurrent subprocesses calling ssh -T [email protected] and 3 concurrent subprocesses signing commit using PIV key.

[AbGuthrie]

This is great, LGTM!