Enforce valid range split keys.

Switched MVCCFindSplitKey to do a simpler scan using the recorded range
stats to determine half-way size and finding the closest key to the
halfway mark that does violate any illegal key ranges.
Add callbacks to gossip.

Support for regexp patterns on keys to have gossip invoke callback
when new infos are added.
Add a split key balance unittest
address feedback
Merge pull request #172 from cockroachdb/spencerkimball/valid-split-keys

Enforce valid range split keys.
Merge pull request #173 from cockroachdb/spencerkimball/gossip-callbacks

Add callbacks to gossip.
Rename Coordinator to TxnCoordSender.

For clarity of purpose. Didn't want this in the last big change.
Merge pull request #176 from cockroachdb/spencerkimball/rename-coordinator

Rename Coordinator to TxnCoordSender.