Merge #12422: util: Make LockDirectory thread-safe, consistent, and fix OpenBSD 6.2 build

1d4cbd2 test: Add unit test for LockDirectory (Wladimir J. van der Laan)
fc888bf util: Fix multiple use of LockDirectory (Wladimir J. van der Laan)

Pull request description:

  Wrap the `boost::interprocess::file_lock` in a `std::unique_ptr` inside the map that keeps track of per-directory locks.

  This fixes a build issue with the clang 4.0.0+boost-1.58.0p8 version combo on OpenBSD 6.2, and should have no effect otherwise.

  Also add a unit test, make the function thread-safe, and fix Linux versus Windows behavior inconsistency.

  Meant to fix #12413.

Tree-SHA512: 1a94c714c932524a51212c46e8951c129337d57b00fd3da5a347c6bcf6a947706cd440f39df935591b2079995136917f71ca7435fb356f6e8a128c509a62ec32

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