Remove synchronized from high-contention methods on RAMFile [LUCENE-7778]

When benchmarking RAMDirectory access via multiple threads the methods `RAMFile::numBuffers` and `RAMFile::getBuffer` show up blocking threads fairly frequently

By removing the `synchronized` keyword from these methods our internal benchmarks show a 2x performance increase under concurrent load.

I don't think removing `synchronized` from these methods is a problem as they are read-only and write access to these fields is not synchronized. #3853 also implies that some ofthe locking on RAMDirectory is not necessary

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Migrated from [LUCENE-7778](https://issues.apache.org/jira/browse/LUCENE-7778) by Steve Mason (@spmason), updated May 17 2017

Locate the RAMFile class and inspect numBuffers and getBuffer, then trace their callers and the unsynchronized field writes. Compare the behavior with the existing RAMDirectory locking and run the concurrent-access benchmarks mentioned in the issue; done means the methods no longer block unnecessarily without introducing unsafe reads.

Written by the indexing model from the issue text.