Ngccc colored coin store

Stores information about colored transaction outputs.

For the sake of simplicity and interoperability, we'll use SQLite database for this, although eventually might switch to something offering higher performance and lower overhead.

Data tuples: (color_id, txhash, outindex, value, label). Label can be used by advanced coloring functions, otherwise it is unnecessary.

Methods:

ColoredCoinStore
   add(color_id, txhash, outindex, value, label)
   remove(color_id, txhash, outindex)
   get(color_id, txhash, outindex)
   get_any(ctxhash, outindex)
   get_all(color_id)