worker/apiaddressupdater: clarify docs
worker/peergrouper: don't call SetAPIHostPorts unless changed

We're spamming the transaction log with SetAPIHostPorts
transactions even when the addresses have not changed.
This change stops that from happening.

Fixes https://bugs.launchpad.net/bugs/1345832
Merge pull request #389 from axw/lp1345832-nochange-nosetapihostports

worker/peergrouper: don't call SetAPIHostPorts unless changed

We're spamming the transaction log with SetAPIHostPorts transactions even
when the addresses have not changed. This change stops that from happening.

Drive-by improvement to apiaddressupdater doc comments, which
were wrong and unhelpful.

Fixes https://bugs.launchpad.net/bugs/1345832