Change BuildMerkleTreeStore to accept transactions.

This commit modifies the BuildMerkleTreeStore function to accept a slice
of btcutil.Tx transactions as opposed to a full block.  This allows more
flexibility when calculating merkle roots since a full block may not be
created yet (particularly when generating blocks that need to be solved in
mining).

Previously, the BuildMerkleTreeStore function accepted a btcutil.Block
because originally the block itself cached the transaction hashes and it
was necessary to have access to the block to make use of the cached
transactions.  However, the code has since been improved such that it
caches transaction hashes directly in each btcutil.Tx.  This means the
code can remain as efficient as before while allowing the individual
transacitons to be passed.