ee567fa8 boldy claimed that it had made MetricRegistryMetricReader
thread-safe. It had not. This commit should actually make it thread
safe. I hope.

One notable improvement is that MetricRegistryMetricReader.findAll()
will no longer contain null values if a metric is removed on another
thread during iteration.

names is now a ConcurrentHashMap to allow it to be safely read and
written without holding a lock.

reverse is a LinkedMultiValueMap  which is not thread-safe. This could
lead to values being lost when concurrent add calls were made. Access
to reverse is now protected by synchronizing on an internal monitor
object.

Calls to containsKey(key) followed by get(key) have been reworked to
only call get(key), this avoids the possibility of the key being
removed after the contains check but before the get.

Closes gh-2590