mock: add Counters, Gauges and Timers methods to get a map of all stats

[charlievieth]

Add Counters(), Gauges() and Timers() methods to stats/mock that return maps
of all the currently stored counters/gauges/timers. This makes it easier to identify
extraneous stats in tests.

This PR also increases test coverage and improves Sink performance when called
in parallel.

$ go test -covermode=atomic
PASS
coverage: 97.5% of statements
ok  	github.com/lyft/gostats/mock	0.016s

benchmark                             old ns/op     new ns/op     delta
BenchmarkFlushCounter-12              50.4          51.9          +2.98%
BenchmarkFlushCounter_Parallel-12     36.5          10.5          -71.23%
BenchmarkFlushTimer-12                52.1          49.8          -4.41%
BenchmarkFlushTimer_Parallel-12       42.0          13.5          -67.86%

benchmark                             old allocs     new allocs     delta
BenchmarkFlushCounter-12              0              0              +0.00%
BenchmarkFlushCounter_Parallel-12     0              0              +0.00%
BenchmarkFlushTimer-12                0              0              +0.00%
BenchmarkFlushTimer_Parallel-12       0              0              +0.00%

benchmark                             old bytes     new bytes     delta
BenchmarkFlushCounter-12              0             0             +0.00%
BenchmarkFlushCounter_Parallel-12     0             0             +0.00%
BenchmarkFlushTimer-12                0             0             +0.00%
BenchmarkFlushTimer_Parallel-12       0             0             +0.00%