ensure sum_finished mutex is always unlocked
Merge pull request #143 from satterly/bugfix/gmetad-unsum-mutex-deadlock

Failure to relase summary mutex is causing gmetad deadlock
Revert "Revert "Improve metric summarization performance under heavy load""

This reverts commit 50a51f2f1464a59c9552d6648222b80d12a0b09e.