Merge Threads unnecessarily block on SerialMergeScheduler [LUCENE-5310]

I have been working on a high level merge multiplexer that shares threads across different IW instances and I came across the fact that SerialMergeScheduler actually blocks incoming thread is a merge in going on. Yet this blocks threads unnecessarily since we pull the merges in a loop anyway. We should use a tryLock operation instead of syncing the entire method?

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Migrated from [LUCENE-5310](https://issues.apache.org/jira/browse/LUCENE-5310) by Simon Willnauer (@s1monw), updated May 09 2016
Attachments: [LUCENE-5310.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-5310/LUCENE-5310.patch) (versions: 7)

Start by locating the SerialMergeScheduler entry point and read how merge requests are pulled while another merge is running. Review the attached LUCENE-5310.patch for context, then verify that incoming merge threads no longer block unnecessarily and that existing merge-scheduler tests still pass.

Written by the indexing model from the issue text.