Make low-s verification mandatory for standard script verification
Make low-s verification mandatory for standard script verification

This change makes a node only accept transactions with low-s
signature encoding for relay and mining, but allows transactions
with high-s signature encoding in mined blocks (no blocks will
be rejected)

Pros:
- If deployed by all miners, this will eliminate this particular
  malleability attack.
- There is no impact on consensus

Cons:
- Wallets that do not implement low-s signature encoding will
  see their transactions be rejected by growing numbers of peers
  and ultimately not be able to get any transaction mined.

Follow ups:
- Eventually, this verification needs to be confirmed through a
  consensus rule (enforcement of BIP62)

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