Support stellar-core protocol 12

[rice2000]

The upcoming stellar-core protocol 12 release will add support for CAP 24 ("Make PathPayment Symmetrical").

To support this change, the following steps are necessary:

• Existing PathPayment operation should be deprecated (it will be renamed to PathPaymentStrictReceive in protocol 12, with behavior unchanged). Consider adding the new operation name immediately as an alias during the transition period.
• Once v12 is voted on and is adopted by the network, a new operation, PathPaymentStrictSend will be available and should be provided. This is expected to happen around the end of October, 2019.
• Horizon will continue to provide the old path_payment operation name in responses until Horizon 0.24. Both types of path_payment operation will be returned with the existing name. In Horizon release 0.24 (targeted for ~10 weeks from now), Horizon will change, and begin to return path_payment_strict_receive and path_payment_strict_send in the type field of responses that refer to operations or payments. You should update client code if necessary (e.g. for validation) and note this for users.

For further information, see the following issues in the SDF Go monorepo:

• stellar/go#1737 - overview and rationale
• stellar/go#1771 - Go SDK work pre-v12 vote
• stellar/go#1772 - Horizon work related to v12
• stellar/go#1773 - Go SDK work post-v12 vote