Keepalive no longer triggered

[jaraco]

It seems that recently, the TCP keepalive option isn't being honored. pmxbot announces the configuring of keepalive during startup, but even if the connection dies, no errors are reported by an attempted keepalive.

[jaraco]

The issue probably emerged with 05ee5f0, where execute_every was disabled. That change was by design because pmxbot was invoking the lower-level .add for scheduling, but it didn't take into consideration that irc itself invokes the execute_every to set up keepalives (and potentially other uses).