NEXUS-6294: Deleted file handles.

While not quite same as NEXUS-6172, a bit different
approach was made here. Writer is kept open during the
lifetime of timeline component, and readers are now
opened per-request. This should be "light" operation
as it's done with applyDeletes=false.

Before, the searcherManager opened a reader at
component creation (nexus boot), and that
reader was kept as is even if nobody touched
feeds (reader keeps a "snapshot" of index frozen
at the point in time where it was open).
Revert "NEXUS-6294: Deleted file handles."

This reverts commit 26a5dbe10f10e73a453c862e763d895b597656aa.
NEXUS-6294: Redoing change

Basically, not doing much, as only making sure that
after purge the deleted file handles are released for sure
(purge would introduce most change to index segments).

Also, not punishing any writer only the "batch" one
that is used only in special circumstances, like
"repair index" on boot, when timeline index is
detected corrupt.

Everything else left intact.
Merge pull request #384 from sonatype/NEXUS-6294-file-handles

NEXUS-6294: Deleted file handles.