Make names of variables and functions consistent in PersistentIndex

[vgkholla]

PersistentIndex.java uses index and index segment interchangeably to refer to index segments. This creates confusion and inconsistency and affects readability. Change PersistentIndex.java and IndexSegment.java so that all variable and function names are consistent and appropriate.

[vgkholla]

There are a few suggestions by @pnarayanan on #488 that can be incorporated.

[vgkholla]

https://github.com/linkedin/ambry/pull/488#discussion_r89040830

[vgkholla]

Something similar applies to IndexSegment where the concept of segment and index is intermingled in the variable names and would benefit from separating those ideas in the naming.