Mainnet updates

[buffrr]

This PR does the following:

• Simplifies space scripts reducing on-chain transaction size (effectively resets testnet4)
• Replace SName type (used to represent a multi-label canonical DNS name) to a simpler SLabel type which only represents a single label.
• Spaces are now alphanumeric only limited to 62 characters.
• Reserve spaces @example, @test and @local
• Adds mainnet-alpha and mainnet chain (activates 871222).
• Update tesetnet4 activation block
• Label dust outputs a bit differently to make coin selection
  for bids safer.
• Add an option to store the raw transaction during indexing
  using --block-index-full useful if you don't want to enable txindex in core.
• Add wallet command to force spend an output (handy for testing)
• Various other clean ups.