Fix baby sitter sorting of the children nodes.

The children nodes are ephemeral sequential nodes, but their prefix are
different. To sort them based on creation order, we have to sort by the
ephemeral sequence number part, not the whole string.
Minor fixes to CoreZK.get*FromChildName() and fix TestBabySitter.
Merge pull request #1278 from VoltDB/babysitter-sort

Fix baby sitter sorting of the children nodes.

r=izzy