Introduce a conflicting tx pool

Summary:
Store a limited quantity of conflicting transactions. This is
currently only used for an RPC but will later be used to vote on these
transactions via avalanche.

Test Plan:

./test/functional/test_runner.py rpc_gettransactionstatus

Reviewers: #bitcoin_abc, bytesofman, PiRK

Reviewed By: #bitcoin_abc, bytesofman, PiRK

Subscribers: tobias_ruck, PiRK, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D16645