Minimise the possibility of transaction loss

[kc1212]

In cases when a leader restarts or a follower restarts or fails to send the pending transactions to the leader, some transactions might be lost as @jeffallen pointed out in https://github.com/dedis/cothority/pull/1329#discussion_r200075174

We should make an effort to minimise transaction loss.

[jeffallen]

Possible reposnes:

1. don't do anything: clients should not trust the conode they give their tx to, should keep track if the tx never arrives in the blockchain and resend.
2. followers store pending txs to stable store asap, and monitor if a tx has arrived in the blockchain. (no protocol change).

This probably is not needed, but keeping to not lose it: What if a v3.1.x conode asks a v3.0.x conode for "CollectTx2", which is a protocol a v3.0.x conode does not know? Because if that's handled right, then we can take this on later, if our solution needs a new protocol.

Decision: we will not do this for v3.

[jeffallen]

Before working on this, we need a clear design, which addresses the question "why do this at all? why not require clients to do this?"