Refactor `DEFAULT_MAX_GENERATED_BLOCK_SIZE` into chain-specific defaults

Summary

We would like to be able to provide chain-specific default mined block sizes. This will allow us to fine-tune the defaults that BCHN ships with to be specific to each chain. Motivation: What is a good default for ScaleNet or RegTest doesn't necessarily work as a global default for MainNet.

This commit removes the global constant DEFAULT_MAX_GENERATED_BLOCK_SIZE since its existence now could potentially be a source of confusion and bugs. The default mined block size now is a chain-specific setting that lives in Consensus::Params and is set-up in chainparams.cpp for each of the 5 chains we support.

Moreover, the Config object manages the user setting as specified in config file or CLI. Two new methods were added to the Config object to get/set the GeneratedMaxBlockSize.

As such, we got rid of checking the gArgs object for -blockmaxsize during critical paths, since that is just wasted CPU cycles (the object must re-parse the string arg each time!).

init.cpp is responsible for setting up Config to respect the chain-specific defaults, as well as for reading in the user-specified setting -blockmaxsize.

Unit tests were updated, as was miner.cpp to use the new config value (rather than read gArgs each time).

Invariants are enforced within the Config object on SetMaxGeneratedBlckSize, (as was already being done with with ExcessiveBlockSize).

Also in this commit: Fixed the -hh help to show chain-specific -excessiveblocksize and -blockmaxsize.

Test Plan

• ninja check check-functional
• Verify bitcoind -hh has correct chain-specific defaults in the help text (for both -blockmaxsize and -excessiveblocksize).