Refactor DatabaseConfiguration

[sfc-gh-anoyes]

Currently there is duplicate state in DatabaseConfiguration. This makes changes to DatabaseConfiguration error-prone, since one must be careful to keep the duplicate state consistent. E.g. there was a bug recently where the order of mutations led to a different final value for getDesiredCommitProxies() (#6645). Further, much of this duplicated state is public so one must look at all references to each member to reason about this instead of just needing to look at the implementation of DatabaseConfiguration.

I propose we make DatabaseConfiguration::{applyMutation, set, clear} the public, non-const interface and make all the member variables private.

Perhaps a pragmatic way to approach this would be to just do this after applying each mutation, or maybe lazily directly before reading a derived value if stale.

[sfc-gh-ahusain]

Also, given totalProxies = grv_proxies + commit_proxies an alternate way to allowing a user to supply configuration could be:

1. Total Proxy Count
2. SplitRatio to divvy proxies CommitProxies:GrvProxies
3. Default Min value for Commit Proxies and Grv Proxies

If we agree to refactor the code in above mentioned way, then user need to supply only above totalProxyCount and splitRatio and the code should be able to compute the right values for CP as well as GRVs without having any issues with ordering of applying confKeys. To me, it is more intuitive way to configuring a cluster (for instance: 80% proxies needs to be CPs etc.)

Further, the min values for CP and GrvProxies should be able to handle invalid input scenarios if needed.