[mempool] Mark mempool import fails that were found in mempool as 'already there'

Summary:
I was investigating the reasons for failed imports in mempool and noticed that LoadMempool() and pwallet->postInitProcess() (for all wallets) are executed concurrently. The wallet will end up importing transactions that LoadMempool() later tries to import; the latter will fail due to the tx already being in the mempool.

Backport of Bitcoin Core PR11062
https://github.com/bitcoin/bitcoin/pull/11062

Test Plan:

make check-all

Reviewers: Fabien, #bitcoin_abc, deadalnix

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4266