DeterministicKeyChain. Creation times in the DeterministicKeyChain can't be persisted to protobuf,
as that structure has no full-blown protobuf equivalent.
This means a couple of DeterministicKeyChain, KeyChainGroup and Wallet factory method variants
that take creation dates have been removed. On the other hand, a convenient
Wallet.fromWatchingKeyB58(params, xpub, creationDate) has been added.
Also adds a test for protobuf-roundtripping watching wallets.
Supposed to fix issue #1209.
The idea of this new field is that if a pending transaction was not announced for a longer time,
it is unlikely to confirm. Thus, the user should be warned and maybe the transaction removed from
the wallet at some point.
* move handling of following keychains into the leading keychain
* move multisig threshold into the leading keychain
* extract MarriedKeyChain from DeterministicKeyChain
Introduced pluggable signers notion. Instances of
TransactionSigner could be added into the wallet, so that they subsequently
applied to transaction to complete it.
Existing signing code (Transaction.signInputs) was refactored into
LocalTransactionSigner, which is always implicitly added to any wallet.
Related pull request: #157
