[Trivial] Restore ReprocessBlocks in main.cpp / Rename DisconnectBlocksAndReprocess to DisconnectBlocks

[lyricidal]

Chengeset:

• move ProcessSpork, GetSporkValue, IsSporkActive, ExecuteSpork and mapSporksActive to CSporkManager
• move Sign, CheckSignature, Relay to CSporkMessage
• move ReprocessBlocks out of sporks to main.cpp / rename DisconnectBlocksAndReprocess to DisconnectBlocks
• rename SporkKey to SporkPubKey
• bugfix: only set strMasterPrivKey if spork signature produced by that key was verified successfully
• few log format changes, cleaned up includes
• Protect CSporkManager with critical section
• bugfix: do not accept sporks with nTimeSigned way too far into the future
• remove unused sporks: 7, 11, 12
• Refactor code to get rid of repeated if/else blocks (introduce CSporkDef)

Mostly based off of:

• Refactor/fix spork dashpay/dash#922
• Protect CSporkManager with critical section dashpay/dash#2213
• Do not accept sporks with nTimeSigned way too far into the future dashpay/dash#2578
• Remove a few sporks which are not used anymore dashpay/dash#2607
• Refactor sporks to get rid of repeated if/else blocks dashpay/dash#2946

Cherry picked from PIVX-Project/PIVX#1000 as these functions will be useful in the future.

ReprocessBlocks function was in spork.cpp which was eliminated before the code was forked.