[RPC][Mining] Extract last Hashspeed calculation

[CaveSpectre11]

Problem

Users want to be able to see their current hashing speed

Solution

Extract the last calculated hashspeed and provide it in getmininginfo. To note, this only pulls the last calculation that was done, and is not a real-time pull. It will be the average from the start of mining to the last calculation, from the time the first generatecontinuous true was run. In other words, this will take into account any 'breaks' taken in mining. e.g. if you have a script that turns off mining for a certain time; the hash speed calculations are still being run; and the break will be calculated into your overall hash rate.

Logic was added to setminingalgo to reset the statistical counters. So after setminingalgo is issued, the statistics will restart on the next generatecontinuous. This can be used to reset your counters even if you're not changing algorithms by setting your mining algo to the same algo:

$ ./veil-cli -regtest getmininginfo
{
[...]
  "hashspeed": 743034,
[...]
}
$ ./veil-cli -regtest setminingalgo sha256d
{
  "success": true,
  "message": "Mining algorithm changed from sha256d to sha256d"
}
$ ./veil-cli -regtest getmininginfo
{
[...]
  "hashspeed": 0,
}

Note that for simplicity sake, the calculation is "hashes per second", so that it can remain a numerical value and users can wrap the use however they wish if they want to convert it to kh/s or mh/s [or whatever they desire]

[CaveSpectre11]

This addresses issue #875

[codeofalltrades]

Looks goods

{
  "blocks": 1002401,
  "currentblockweight": 43832,
  "currentblocktx": 2,
  "algorithm": "sha256d",
  "difficulty": 18.71628533785223,
  "networkhashps": 23.47822805181118,
  "hashspeed": 351949,
  "pooledtx": 2,
  "chain": "main",
  "warnings": "This is a pre-release test build - use at your own risk - do not use for mining or merchant applications"
}

{
  "blocks": 1002409,
  "currentblockweight": 31364,
  "currentblocktx": 1,
  "algorithm": "randomx",
  "difficulty": 0.003405607281592146,
  "networkhashps": 23.28523685595233,
  "hashspeed": 43,
  "pooledtx": 1,
  "chain": "main",
  "warnings": "This is a pre-release test build - use at your own risk - do not use for mining or merchant applications"
}```

[codeofalltrades]

ACK ea18c5a