mempool: Fix missing locking in CTxMemPool::check(…) and CTxMemPool::setSanityCheck(…)

Summary:
This is a backport of Core PR11689

Fix missing locking in CTxMemPool::check(const CCoinsViewCache *pcoins)

• reading variable 'mapTx' requires holding mutex 'cs'
• reading variable 'mapNextTx' requires holding mutex 'cs'
• reading variable 'nCheckFrequency' requires holding mutex 'cs'

Fix missing locking in CTxMemPool::setSanityCheck(double dFrequency)

• writing variable 'nCheckFrequency' requires holding mutex 'cs'

Add Clang thread safety analysis annotations

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3307