Implemented index cache mode for both BAI & BTI formats

Implemented index cache mode for both BAI & BTI formats

 * Client code can now decide between 3 index cache modes:
   Full : save entire index data in memory
   Limited (default) : save only index data for current reference
   None : save no index data - only load data  necessary for a single-

 * Required a major overhaul to BamIndex interface and derived classes.
   Lots of refactoring to move common code up to BamIndex.
   Derived classes now share much of the same method names &
organization.  Only implementation details differ, as needed.

 * Miscellaneous: moved BAMTOOLS_LFS definitions into BamAux.h & cleaned
up BGZF.h