Surface merge policy and related settings

[chriseldredge]

The default configuration of the underlying IndexWriter is difficult to customize when allowing Lucene.Net.Linq to instantiate IndexWriter upon first usage. This leads to some undesired performance results with regards to how and when index segments are merged. For clients that want better control of these policies, Lucene.Net.Linq should make it easier to select a merge policy implementation and related settings like MergeFactor, MinMergeMB, MaxMergeMB, etc.

This would allow clients to prevent too many deleted documents from accumulating.

See #72 for origin of this issue.

[chriseldredge]

This is currently possible by subclassing LuceneDataProvider and overriding GetIndexWriter.

Perhaps adding some more properties to LuceneDataProviderSettings would be a good approach.

[chriseldredge]

Commit 67b93d9

[chriseldredge]

Released in https://github.com/themotleyfool/Lucene.Net.Linq/releases/tag/v3.5.0