Merge loops in CommunicateTxCurve

Save some time in bigger sized blocks. Tested on hard limit of
blockmaxsize. Also add hard limit of blockmaxsize to the miner and
limitfreerelay until https://github.com/conformal/btcd/pull/160 gets
settled.
Yes, i know. But it's a corner case and i don't want to add another specific channel to every select about it. Maybe i shall use ```com.fail```.