Allow for dynamic pool changes

Current situation

Pools are read from SSHServiceConfig or SSH_CHANNEL_POOLS at startup time. This implies that adding SSH channels requires a service restart.

Desired outcome

SSHCHANNEL_POOLS, if defined, (of SSH_CHANNEL_POOLS, if the former is not defined) should be watched and changes should be dynamically reloaded.

All pools in this file will be accessible from the running context.