NEXUS-5468: HTTP connection leak in case of LSEx

Fix for LRS to always clean up PreparedContentLocator instances
and a UT verifying this happens in both case, in "norma" but
also in "early unexpected" exception case.
NEXUS-5468: Applying original parts

Instead of introducing new custom method. PreparedContentLocator
made plain Closeable. Using guava Closeables to
make it closed.
NEXUS-5468: Generalizing the solution

To not be specific for PreparedContentLocator, but instead
make them proper "Closeable"s. Same for FS RLS, it cares about
Closeable, not about given ContentLocator implementation.
Merge pull request #820 from sonatype/nexus-5468-connection-leak

[REVIEW] NEXUS-5468: HTTP connection leak in case of LSEx