Make test utility class MemHostDataSource thread-safe

MemHostDataSource is used in certain unit tests.

RADIUS MT unit tests required MemHostDataSource to be thread-safe, so the TestHostCache that derives it overrode all its methods and added a lock_guard to each.

To avoid this boilerplate code, ideally, MemHostDataSource should be made thread-safe itself.

This was not done at the time due to lack of time before the release.

When this is done, remember to remove the overridden methods from TestHostCache:

• premium/src/hooks/dhcp/radius/tests/access_unittests.cc
• premium/src/hooks/dhcp/radius/tests/accounting_unittests.cc

@fdupont says

Note the mutex must be at most protected.