Replace inefficient new Long(long) constructor to silence FindBugs

Replace inefficient new Long(long) constructor to silence FindBugs

FindBugs keeps warning us that new Long(long) is inefficient because
it doesn't permit using cached values in the -128..127 range.

We now use a custom Map implementation which supports primitive long
as the hash key, rather than requiring boxing for java.util.HashMap.
This removes the issue FindBugs was identifying.

This version performs slightly better than before.
index-pack on linux-2.6:

  parent commit:  2m58.611s
  this commit  :  2m57.068s

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
CC: Yann Simon <yann.simon.fr@gmail.com>
CC: Matthias Sohn <matthias.sohn@sap.com>
Signed-off-by: Robin Rosenberg <robin.rosenberg@dewire.com>