hooks for consensus / network parameters

[jchappelow]

While this code in #1213 does the merge of updates into the active parameters, it's really not so simple for every parameter. e.g. changing DB owner needs some other actions almost certainly. MigrationStatus, while mutable and grouped with the other mutable params, should not be changed via this mechanism, right? Leader can be changed in this updates mechanism, but like DB owner, there would have to be other actions or at least the change detected by CE. There can be higher level code that detects changes, or there can be some hooks that get called here, although I'm haven't explored how to define and inject them.

Originally posted by @jchappelow in #1213 (comment)