Abstract out SignatureChecker
Make signature cache optional
Replace SCRIPT_VERIFY_NOCACHE by flag directly to checker
Don't translate state.Abort() messages

There is only one message passed to AbortNode() that makes sense to
translate to the user specifically: Disk space is low. For the others
show a generic message and refer to debug.log for details.

Reduces the number of confusing jargon translation messages.
Update English translation
Make comptool more deterministic
Merge pull request #5035

c9c51cb Make comptool more deterministic (Matt Corallo)
Merge pull request #4890

e790c37 Replace SCRIPT_VERIFY_NOCACHE by flag directly to checker (Pieter Wuille)
5c1e798 Make signature cache optional (Pieter Wuille)
c7829ea Abstract out SignatureChecker (Pieter Wuille)
Merge pull request #5036

eb6b3b2 Update English translation (Wladimir J. van der Laan)
b9b2e3f Don't translate state.Abort() messages (Wladimir J. van der Laan)
Headers-first synchronization

Many changes:
* Do not use 'getblocks', but 'getheaders', and use it to build a headers tree.
* Blocks are fetched in parallel from all available outbound peers, using a
  limited moving window. When one peer stalls the movement of the window, it is
  disconnected.
* No more orphan blocks. At all. We only ever request a block for which we have
  verified the headers, and store it to disk immediately. This means that a
  disk-fill attack would require PoW.
* Require protocol version 31800 for every peer (released in december 2010).
* No more syncnode (we sync from everyone we can, though limited to 1 during
  initial *headers* sync).
* Introduce some extra named constants, comments and asserts.
RPC additions after headers-first
Remove CheckMinWork, as we always know all parent headers
Improve getheaders (sending) logging
Better logging of stalling
Add height to "Requesting block" debug