Make os_cache_max and os_cache_dirty_max reconfigurable

Currently the os_cache_dirty_max table setting is not able to be changed after it is set in session->create(). With the work in WT-4406, it may be desirable to modify that value after creation. There appears to be no technical reason it cannot be reconfigurable. My guess is that is simply wasn't ever needed before in that way.

This work should make os_cache_dirty_max (and os_cache_max while we're in there) able to be reconfigured via session->alter().

My initial thoughts are that changing its definition in dist/api_data.py and write a test or two to verify it is working as expected are all that are needed.