Make mmap mode more reliable by forcing GC at the correct spot

Make mmap mode more reliable by forcing GC at the correct spot

ObjectLoaders can defer the bulk of their data access to the
getCachedBytes() method invocation, rather than at the time
of their construction.  That means there's code paths into
the mmap allocator which aren't protected by the "try GC and
try again" within openObject, resulting in some random crashes
due to mmap failures.

The right place to check for mmap failure and retry is within
WindowedFile.loadWindow, where we actually invoke the mmap call.
If the call fails, we need can immediately retry it, and if it
fails again, we can temporarily degrade to the byte[] variant
until the JVM is able to evict and GC enough space.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Robin Rosenberg <robin.rosenberg@dewire.com>