Split ranges along accounting and zone configuration prefix boundaries.

Each store registers a gossip callback for accounting and zone config
maps. On callback, the zone does a binary search using the rangesByKey
sorted slice to find possible ranges which might be intersected by
config map prefixes. On possible hits, the range is dissected along
prefix boundary lines and if there's a necessary split, the range is
split and the process retried.

This allows new zones or accounting prefixes to be specified and ranges
will automatically be split to fit.
Configuration groups cannot span ranges. The illegality of splitting mid-config group is enforced in storage/engine/mvcc.go:803 at IsValidSplitKey(). I'm adding a comment here to that effect.