Adding custom memstore implemenation for indexing

We basically wrap a KeyValueSkipListSet, just
like a regular MemStore, except we are (1) not
dealing with (a) space considerations or (b) a snapshot
set, and (2) ignoring memstore timestamps in favor of
deciding when we want to overwrite keys based on how we
obtain them.

We can ignore the memstore timestamps because we know that
anything we get from the local region is going to be MVCC
visible - so it should just go in. However, we also want
overwrite any existing state with our pending write that
we are indexing, so that needs to clobber the KVs we get
from the HRegion. This got really messy with a regular
memstore as each KV from the MemStore frequently has a
higher MemStoreTS, but we can't just up the pending KVs'
MemStoreTs b/c a memstore relies on the MVCC readpoint,
which generally is < Long.MAX_VALUE.

By realizing that we don't need the snapshot or space
requirements, we can go much faster than the previous
implementation. Further, by being smart about how we
manage the KVs, we can drop the extra object creation
we were doing to wrap the pending KVs (which we did
previously to ensure they sorted before the ones we got
from the HRegion).