Performance improvements for FastCharStream [LUCENE-5723]

Hello from the .NET land,

A user of ours has identified an optimization opportunity, although minor I think it points to a valid point - we should avoid using exceptions from controlling flow when possible.

Here's the original ticket + commits to our codebase. If this looks valid to you too I can go ahead and prepare a PR.

https://issues.apache.org/jira/browse/LUCENENET-541
https://github.com/apache/lucene.net/commit/ac8c9fa809110ddb180bf7b2ce93e86270b39ff6
https://git-wip-us.apache.org/repos/asf?p=lucenenet.git;a=blobdiff;f=src/core/QueryParser/QueryParserTokenManager.cs;h=ec09c8e451f7a7d1572fbdce4c7598e362526a7c;hp=17583d20f660fdb6e4aa86105c7574383f965ebe;hb=41ebbc2d;hpb=ac8c9fa809110ddb180bf7b2ce93e86270b39ff6

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Migrated from [LUCENE-5723](https://issues.apache.org/jira/browse/LUCENE-5723) by Itamar Syn-Hershko, updated Dec 09 2014

Start with the linked Lucene commit and the diff for src/core/QueryParser/QueryParserTokenManager.cs, then compare the corresponding current implementation. Verify that the proposed FastCharStream optimization preserves parser behavior and identify existing tests that cover it; done means the behavior remains covered without using exceptions for normal control flow.

Written by the indexing model from the issue text.