WindowCache: add option to use strong refs to reference ByteWindows

WindowCache: add option to use strong refs to reference ByteWindows

Java GC evicts all SoftReferences when the used heap size comes close to
the maximum heap size. This means peaks in heap memory consumption can
flush the complete WindowCache which was observed to have negative
impact on performance of upload-pack in Gerrit.

Hence add a boolean option core.packedGitUseStrongRefs to allow using
strong references to reference packfile pages cached in the WindowCache.

If this option is set to true Java gc can no longer flush the
WindowCache to free memory if the used heap comes close to the maximum
heap size. On the other hand this provides more predictable performance.

Bug: 553573
Change-Id: I9de406293087ab0fa61130c8e0829775762ece8d
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>