Make multisignature configurable

[sfontanach]

(needs feasibility check + more details)

Currently a bazo transaction needs 2 signatures in order get approved.
This is represents a forced 2-of-2 transaction type which is not suitable for all cases.

It is desirable having the possibility of deciding how many signatures are needed for a specific transaction.

See

• https://en.bitcoin.it/wiki/Multisignature
• https://bitcoinmagazine.com/articles/multisig-future-bitcoin-1394686504/

for inspiration.