CBL-1440: Fix FLSlice memory leak

[bmeike]

This commit contains two changes:

1. FLSliceResult now implements AutoCloseable. This is Java's contract that says "you must call close() on this, when you are done with it"
   Closing a core-owned object and failing to close a bindings-owned object just generate warnings. They no longer cause exceptions to be thrown.
2. The FLSliceResult heap block allocated in the JNI is now freed. There are two cases:
   a) Java holds ownership of the FLSR: the allocated block is freed when Java frees is companion.
   b) Java passes ownership of the FLSR to Core: the allocated block is freed before the FLSR is returned to Core

[borrrden]

The switch of the boolean naming scared me at first but there is only one place that uses the isCoreOwned bool as true from what I can tell, so it is doing what it should.