mempool: apply rule of 5 to epochguard.h, fix compiler warnings

Summary: This is a backport of core#22481

Test Plan:
ninja

Tested with clang15, before and after the fix. The commit fixes the -Wdeprecated-copy warning.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11318