Tachyon: Add an atomic_bool `tachyonLatched` to CTxMemPool
Summary
Tachyon activation will involve only mempool acceptance and other non-consensus changes. As such, it was discussed in Slack that perhaps activating Tachyon once and latching it to true would be desired.
This MR adds such a latch. MRs such as !1114 (closed) and !1115 (merged) can be rebased to make use of this latch.
Test Plan
This MR simply adds the boolean as well as logic to latches it to true once the first time through ATMP when the chain tip's MTP is after the tachyon activation time. MR !1114 (closed) makes use of this mechanism.
- review
ninja all check