wallet: Add missing cs_wallet/cs_KeyStore locks to wallet

Summary:
Add missing wallet locks:

• Calling the function GetConflicts(...) requires holding the mutex

cs_wallet

• Calling the function IsSpent(...) requires holding the mutex

cs_wallet

• Accessing the variables mapKeys and mapCryptedKeys requires holding

the mutex cs_KeyStore

• Accessing the variable nTimeFirstKey requires holding the mutex

cs_wallet

• Accessing the variable mapWallet requires holding the mutex cs_wallet
• Accessing the variable nTimeFirstKey requires holding the mutex

cs_wallet

Backport of core PR11634.

Test Plan:
With Clang as a compiler:

cmake -GNinja .. -DCMAKE_BUILD_TYPE=Debug -DENABLE_WERROR=ON
ninja check check-functional

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5063