[svn-r15049] This is an interrim commit of metadata cache journaling mods.

I now have substantial tests for this code -- enough (I hope) for
Mike M. to get started.  However, the code is my no means fully tested.
I don't expect any obvious problems, but there are probably quite a few
relatively subtle bugs remaining.  I'll be chasing these in the next
week.

For an example of setting up the cache to journal, see 
setup_cache_for_journaling() in test/cache2_journal.c 

Warnings:

1) For now, only enable journaling at file creation time -- code to
   do this after the file is opened exists, but it hasn't been tested.

2) Right now the journal logging code is very inefficient, so expect
   things to run slowly until Mike M. checks in his changes to address
   this problem.

3) I have not checked in exemplar journal output files pending a fix 
   another minor bug in the journal logging code.  Until then, the
   journal tests create exemplars and then test against them -- a poor
   way to find errors.

4) The USE_CORE_DRIVER has been moved to cache2_common.h.

5) When USE_CORE_DRIVER is FALSE, cache2_journal runs VERY slowly on
   some system (i.e. 4 hours on Phoenix) -- but it runs fast on Kagiso
   (~10 minutes).  Don't know why, but would guess that the quantity
   of RAM on the system has much to do with it.

Tested serial debug on Phonenix, and parallel debug on Kagiso

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