Remove transaction storage functionality from the database

There is no need to store transactions independently as they will be
stored as a part of a block they are included into. If the server
dies with some transactions before they were mined, it is no big loss
as somebody else will include them.
Extract key & block storage implementations into separate source files
Minor Makefile tweak
Implement functionality for storing 'scraps' in the database
Initial name registrar draft
The database should store a link to the next block
Report number of transactions a miner is mining
Coordinate block mining better when it comes to last blocks
Filter out mined transactions