BamMultiReader data structure rewrite

BamMultiReader data structure rewrite

Rewrite to improve performance of the MultiReader on large sets of
files.  Move tracking of readers, alignments, and positions from several
decoupled vectors into a single multimap, allowing rapid acquisition of
the lowest 'current' alignment among the set of open readers.  Expect
some performance boost when running the MultReader on large numbers of
files, as prior to this rewrite each alignment required roughly 3 x N
ops (where N is the number of files) checking all these vectors.