Reject transactions with excessive numbers of sigops
Reduce maximum coinscache size during verification

Due to growing coinsviewcaches, the memory usage with checklevel=3
(and standard settings for dbcache) could be up to 500MiB on a
64-bit system. This is about twice the peak during reindexing,
unnecessarily extending bitcoind's memory envelope.

This commit reduces the maximum total size of the caches used during
verification to just nCoinCacheSize, which should be the limit.
Merge pull request #4675

ea100c7 Reduce maximum coinscache size during verification (Wladimir J. van der Laan)
Merge pull request #4150

4fad8e6 Reject transactions with excessive numbers of sigops (Peter Todd)
Revert "Reject transactions with excessive numbers of sigops"

This reverts commit 4fad8e6d831729efa1965fa2034e7e51d3d0a1be.
Don't poll showmyip.com, it doesn't exist anymore

Fixes #4679.

This leaves us with only one candidate, checkip.dyndns.org.
GetMyExternalIP should be phased out as soon as possible.
Merge pull request #4682

c33b983 Don't poll showmyip.com, it doesn't exist anymore (Wladimir J. van der Laan)
Reapply: Reject transactions with excessive numbers of sigops

Reverting was based on a misunderstanding, it appears.

Github-Pull: #4150