Take the cache size into account when calculating the maximum size of a page in-memory.  Use that calculation in the decision to deepen internal pages.

refs #486
Implement review feedback for #980.
typo
Make the upper bound on the maximum page size work with shared caches: if the cache size hasn't been set when creating the table, ignore it.

refs #980
Merge pull request #980 from wiredtiger/deepen-small-cache

Take the cache size into account when calculating the maximum size of a page in-memory