Merge #11904: Add a lock to the wallet directory

Summary:
2f3bd47 Abstract directory locking into util.cpp (MeshCollider)
5260a4a Make .walletlock distinct from .lock (MeshCollider)
64226de Generalise walletdir lock error message for correctness (MeshCollider)
c9ed4bd Add a test for wallet directory locking (MeshCollider)
e60cb99 Add a lock to the wallet directory (MeshCollider)

Pull request description:

Fixes https://github.com/bitcoin/bitcoin/issues/11888, needs a 0.16 milestone

Also adds a test that the lock works.

https://github.com/bitcoin/bitcoin/pull/11687 will probably rework this to a per-wallet lock instead of just the walletdir, but this fixes the current issue

Tree-SHA512: 98e52d67f820e3b8f919cf361ffbb7d928f1bd67603e0ed26c5076ea02d9b3a90c3535ddf7329f3b88171396fa28dd3c87adab3577a8a217bd1e4247bda99138

Backport of Core PR 11904
https://github.com/bitcoin/bitcoin/pull/11904/files
Completes T604
Depends on D2807

Test Plan:
make check
test_runner.py wallet_multiwallet

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D2808