Before this commit, snapshot stored information of distributed in-progress
transactions populated during snapshot creation and its corresponding localXids
found during tuple visibility check later (used as cache) by reverse mapping
using single tightly coupled data structure DistributedSnapshotMapEntry. Storing
the information this way possed couple of problems:

1] Only one localXid can be cached for a distributedXid. For sub-transactions
same distribXid can be associated with multiple localXid, but since can cache
only one, for other local xids associated with distributedXid need to consult
the distributed_log.

2] While performing tuple visibility check, code must loop over full size of
distributed in-progress array always first to check if cached localXid can be
utilized to avoid reverse mapping.

Now, decoupled the distributed in-progress with local xids cache separately. So,
this allows us to store multiple xids per distributedXid. Also, allows to
optimize scanning localXid only if tuple xid is relevant to it and also scanning
size only equivalent to number of elements cached instead of size of distributed
in-progress always even if nothing was cached.

Along the way, refactored relevant code a bit as well to simplify further.