Compilation: don't warn about using Sun proprietary APIs

E.g. we use Unsafe in quite a few places and this generates lots of
warnings, which we now suppress using the undocumented
-XDignore.symbol.file command-line option to javac.

Closes #6423
FileSystem: Use XNativeFSLockFactory instead of the buggy Lucene 4.8.1 version

There is a pretty nasty bug in the lock factory we use that can cause
nodes to use the same data dir wiping each others data. Luckily this is
unlikely to happen if the nodes are running in different JVM which they
do unless they are embedded.

See LUCENE-5738

Closes #6424