use structured transaction in store and mempool

[jchappelow]

Both the block store and mempool types were still using opaque transctions ([]byte) from the early version of the p2p demo. This updates both to use *types.Transaction.

This change stops short of updating the Block type to have a Txns []*Transaction field instead of Txns [][]byte, but that change should also be made. This can wait until the block data / chain RPCs are complete. It will also require updating the block serialization code, although the effective binary serialization will be identical.

This also updates the mempool implementation and interface:

• internally there is a FIFO queue so that PeekN will return those at the front of the queue. The maps still remain there for fast indexing and existence checks.
• add a Remove method that is the same as Store(txid, nil)

[charithabandi]

LGTM! Merge if you have no other changes.