Don't hold the underlying file/channel open for longer than necessary.

When loading from a database, once read and/or mapped the file/channel can be safely closed.
(Even when using MEMORY_MAPPED, once we have the mapped buffer the original file can be safely closed without affecting the buffer.)
The [documentation for `RandomAccessFile.close()`](http://docs.oracle.com/javase/8/docs/api/java/io/RandomAccessFile.html#close--) has included the following since Java 1.4:

> If this file has an associated channel then the channel is closed as well.