Speeding up IndexMemStore, removing ExposedMemStore.

Lots of time was spent creating a config in IndexMemStore that we didn't need - removing it.
Also, removing the timetracker stuff - we don't care about whether or not to get a scanner
for the store, given a timerange - we always scan the store.

Additionally, removing ExposedMemStore since we are using our own impl now.
Switching ValueGetter to use ImmutableBytesPtr.

There is a bunch of overhead associated with copying byte[]s. We now just return ImmutableBytePtrs
and store ptrs, when at all possible.