NRT Caching Dir to allow for exact memory usage, better buffer allocation and "global" cross indices control [LUCENE-3425]

A discussion on IRC raised several improvements that can be made to NRT caching dir. Some of the problems it currently has are:

1. Not explicitly controlling the memory usage, which can result in overusing memory (for example, large new segments being committed because refreshing is too far behind).
2. Heap fragmentation because of constant allocation of (probably promoted to old gen) byte buffers.
3. Not being able to control the memory usage across indices for multi index usage within a single JVM.

A suggested solution (which still needs to be ironed out) is to have a BufferAllocator that controls allocation of byte[], and allow to return unused byte[] to it. It will have a cap on the size of memory it allows to be allocated.

The NRT caching dir will use the allocator, which can either be provided (for usage across several indices) or created internally. The caching dir will also create a wrapped IndexOutput, that will flush to the main dir if the allocator can no longer provide byte[] (exhausted).

When a file is "flushed" from the cache to the main directory, it will return all the currently allocated byte[] to the BufferAllocator to be reused by other "files".

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Migrated from [LUCENE-3425](https://issues.apache.org/jira/browse/LUCENE-3425) by Shay Banon (@kimchy), 1 vote, updated May 09 2016

Start by understanding the NRT caching directory and its proposed BufferAllocator, including the wrapped IndexOutput and main-directory flush path described in the issue. Determine how allocation caps, cross-index sharing, buffer reuse, and exhaustion-triggered flushing should work; done means the design is agreed and these behaviors are covered.

Written by the indexing model from the issue text.