Add NamedLatch to statsd

This is a sychronizing primitive that is similar to a latch, but a
thread must count down with an identifier. It will be used to make sure
the boot complete, uid map, and all pullers signals are received before
triggering the bucket split.

The latch's countDown operation takes in a string identifier, so that if
the same operation happens twice, it is only counted once.

Bug: 144099206
Test: atest statsd_test

Change-Id: I261a3e50eabbc4998ca30ddf2d67a9a1e788911e