Support partial syncing of addresses in wallet format.

This change reappropriates the unused `last block` field from Armory's
wallet format to hold the block chain height for a partially synced
address, that is, an address that has been partially synced to
somewhere between its first seen block and the most recently seen
block.  The wallet's SyncHeight method has been updated to return
partial heights as well.

The actual marking of partially unsynced address from a rescan
progress update is not implemented yet.