Don't attempt mempool entry for wallet tx on start if already in mempool

Summary:

Mempool loads first, wallet second. Second attempt fails, marking that
transaction !fInMempool. Those funds will disappear until confirmation
is reached.

Backport of core PR11839
https://github.com/bitcoin/bitcoin/pull/11839/files

Test Plan:

./test/functional/test_runner.py mempool_persist

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3425