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.
Adding covered columns back to covered set when found.

We had the infrastructure to track the columns that have already been loaded, but
never actually updated the set of columns that were loaded. This meant that we
reloaded the same columns over and over, even though they had already been loaded