Longer term workaround for chainstate corruption from negative versions.

[gmaxwell]

This also makes negative transaction versions non-standard.

This avoids an issue triggered in block 256818 where transactions with
negative version numbers were incorrectly serialized into the UTXO set.

On restart nodes detect the inconsistency and refuse to start so long as
a block with these transactions is inside the self-consistency check
window, logging "coin database inconsistencies found". The software
recommends reindexing, but reindexing does not correct the problem.

This should be fixed by changing the chainstate serialization, but
working around it seems harmless for now because the version is not
used by any network rule currently.

A patch free workaround is to start with -checklevel=2 which skips
the consistency checks, but the IsStandard change is important for
miners in order to protect unpatched nodes.

[BitcoinPullTester]

Automatic sanity-testing: PASSED, see http://jenkins.bluematt.me/pull-tester/f8b7aa862519ab2efd1ce327d2ed4bea1325dc11 for binaries and test log.
This test script verifies pulls every time they are updated. It, however, dies sometimes and fails to test properly. If you are waiting on a test, please check timestamps to verify that the test.log is moving at http://jenkins.bluematt.me/pull-tester/current/
Contact BlueMatt on freenode if something looks broken.

[gavinandresen]

ACK: tested on OSX.

[laanwj]

ACK: works for me on Ubuntu 12.04 64 bit

[wtogami]

https://github.com/gmaxwell/bitcoin/commit/f8b7aa862519ab2efd1ce327d2ed4bea1325dc11
Testing backport to litecoin-0.8.4.1. Assuming I did it correctly, your bitcoin-0.8.x backport will be slightly different from this.

[jgarzik]

ACK

[wtogami]

ACK

[runeksvendsen]

I can confirm that this patch fixes the issue for me, and tests run without errors.

[sipa]

ACK code changes; didn't test.