Globals: Remove a bunch of Params() calls from main.cpp:

1. Chainparams: Explicit CChainParams arg for main:

-AcceptBlock
-AcceptBlockHeader
-ActivateBestChain
-ConnectTip
-InitBlockIndex
-LoadExternalBlockFile
-VerifyDB parametric constructor

2. Also pickup more Params()\. in main.cpp

3. Pass nPruneAfterHeight explicitly to new FindFilesToPrune() in main.cpp