This commit uses the now created PSBTWalletMixin and additionally
creates a SNICKERWalletMixin, and adds a SNICKERReceiver object
to jmclient. A test of the end to end workflow of create and then
co-sign a SNICKER coinjoin as per the draft BIP is in test_snicker.
Additional changes:
updated python-bitcointx dependency to >=1.0.5
Minor refactoring of callbacks in tests and additional redeem
script checks to PSBTWalletMixin.sign_psbt.
Note that this work replaces #403 .
Adam Gibson