Rotating request journal

This is a patch that adds a command line and WAR file parameter for setting the size of the request journal. If the parameter is not set the journal acts as usual (no limitation on the size). If the parameter is set the request journal is restricted to the given number of entries. If new entries are added old entries are discarded.

This is a compromise between having a complete request journal and not having any (DisabledRequestJournal).

Everything is extensively tested (I created some new tests where appropriate) and I updated the documentation. I executed "gradle test" and everything seems to be ok.

Once this gets merged I think I'm gonna write an admin request for dynamic modification of the request journal size. The code is already in there, the functionality is just not exposed yet.

PS: I also fixed a bug where files containing stub mappings are not closed after reading. This caused some unit tests to fail...