@michaelcahill 
I went part way through implementing this.. It means that all users of the shared cache need to know and specify the size of the cache. I guess it's likely that they will, but I can imagine a use case where there is a single master that sets up the shared cache - then other connections just join in without knowing the details.

I also quite like the idea of being able to parse a configuration string with all settings in it. For example an application may go through all of the configuration settings and setup defaults for them all in their application - disallowing disparate pairs of settings complicates that for applications. Or an application like ```wtperf``` which appends user configuration strings to default configuration strings - the default may use a non-shared cache, but a user may want to override that and setup a shared cache. (I know this one is excluded via our documentation).