[RPC] Reworked getnetworkhashps with tweaks to -mine and getmininginfo

[CaveSpectre11]

[Problem]
#340 - getnetworkhashps seems to be unrelated to actual network hash
#32 - getmininginfo has vastly changing difficulties

[Root Cause]
As greatly discussed in #340, the getnetworkhashps never accurately reflected PoW vs. PoS; as if one of the blocks was a PoS block, and one was a PoW block, the average calculated would vary wildly. Likewise, getmininginfo would only report on the last block, which gets even crazier with 4 different block types.

[Solution]
First was a need to get your current mining algorithm; this is a separate commit. The original code was getting the mining algorithm only when generation was started; so the startup flag sat unused until that time. This also had the side effect of not realizing you would be mining randomX, until you started mining, due to a typo in the -mine flag [e.g. sha256 instead of sha256d. By changing the code to read the flag on startup, it opened the ability to sanity check it on startup; so users using an incorrect flag will now be told so. This also opens the ability to switch mining algorithms without stopping the daemon [although that's for a future PR, as care is needed to make sure you're told to stop mining before switching algorithms].

% veild -mine=sha256
Error: Invalid mining algorithm: sha256

With that out of the way, getnetworkhashps was expanded to still function as would be expected, but instead of just randomly using whatever blocks it found; it now focuses on only the blocks related to the currently set algorithm. e.g. if you want the average network hashes per second over the last 120 blocks [the default] it will be between the last 120 blocks of the specific algorithm you're configured for. An algorithm parameter was added so that you can specify a different algorithm if you chose, including the historical xr16t, or if you care about PoS. Note that this is actually calculating the change in difficulty over time, and not hashes. That research was also out of scope for this PR, and if the logic needs to change it will be a separate PR.

The output to getnetworkhashps is also included in getmininginfo, which in testing revealed that getmininginfo displays the difficulty for the last block; and not the difficulty for the algorithm that you're actually mining. So that logic was changed to report the difficulty for whatever algorithm you're set to mine. An 'algorithm' field was also added to the output so it can be clear what algorithm the difficulty relates to:

$ veil-cli getmininginfo
{
  "blocks": 988984,
  "currentblockweight": 0,
  "currentblocktx": 0,
  "algorithm": "progpow",
  "difficulty": 174.107098381071,
  "networkhashps": 22.83160812508121,
  "pooledtx": 2,
  "chain": "main"
}

$ veil-cli getmininginfo
{
  "blocks": 988986,
  "currentblockweight": 0,
  "currentblocktx": 0,
  "algorithm": "sha256d",
  "difficulty": 19.31658925581963,
  "networkhashps": 23.3181708549265,
  "pooledtx": 1,
  "chain": "main"
}

Given the availability of all the difficulties with the getblockchaininfo command, a parameter to get mining info for a different algorithm was not added.

[codeofalltrades]

0807f52
One question about logic.

[codeofalltrades]

ACK 0807f52