Prevent SSH connection restarts

Set a high idle timeout on the sshkit connection pool. This will
reduce the incidence of re-connection storms when a deployment has been
idle for a while (e.g. when waiting for a docker build).

The default timeout was 30 seconds, so we'll enable keepalives at a
30s interval to match. This is to help prevent connections from being
killed during long idle periods.

lib/mrsk/commander.rb

@@ -143,6 +143,7 @@ class Mrsk::Commander
   private
     # Lazy setup of SSHKit
     def configure_sshkit_with(config)
+      SSHKit::Backend::Netssh.pool.idle_timeout = config.sshkit_pool_idle_timeout
       SSHKit::Backend::Netssh.configure do |sshkit|
         sshkit.max_concurrent_starts = config.sshkit_max_concurrent_starts if config.sshkit_max_concurrent_starts
         sshkit.ssh_options = config.ssh_options