Create configurable Lucene segment size option

There is a need to allow for configurable Lucene segment sizes as this would improve performance on large (ie: 700,000 issues) instances.

Currently it is possible for each individual Lucene segment to get as large at 5 gigs, it may aid to performance considering disk access, merging overheads, and search speeds to allow for more but smaller segment sizes as on option for large instance customers.

Lucene allows for this option already, though JIRA needs to make it user configurable.