Misc DB fixes (5.1DEV__17)

This combines a number of small DB changes before the next round of large breaking modifications:

 * Fix inplace import of read-only files
    * @sbesson tracked down that inplace import of files from squig had been broken since 5.1.0-m3. This was caused by the "rw" flag to `RawFileStore` which caused an exception during `java.io.nio.FileChannel.size()`.
 * http://trac.openmicroscopy.org.uk/ome/ticket/12610
    * Recently reported bug in which long host names overrun the 256 character limit of the `node.conn` column. This simply widens the value. Reproducing this is difficult at best.
 * https://trac.openmicroscopy.org.uk/ome/ticket/11084
   * Deletions of filesets with a large number of files were painfully slow, even after the main database action had taken place. This attempts to correct that by adding database indexes and reducing the overall number of transactions.

Primary testing focus should be on the speed and accuracy of file deletions. There should be no change at the level of graph deletions, only when cleaning up the files on disk after graph deletion. This means that `bin/omero admin cleanse` should show none of the files that were intended to be deleted and `bin/omero delete /Fileset...` should print no warnings. /cc @ximenesuk @pwalczysko @mtbc
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