Add AssertLockHeld assertions in CWallet::ListCoins

Summary:

• Add AssertLockHeld assertions in CWallet::ListCoins
• Add EXCLUSIVE_LOCKS_REQUIRED to CWallet::ListCoins

Suggested by MarcoFalke <falke.marco@gmail.com> in
https://github.com/bitcoin/bitcoin/pull/10605#issuecomment-417643535

This is a backport of Core PR10605

Test Plan:

• Build with clang and make sure there are no warning related to locks.
• Build in debug mode and run the extended test suite.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4170