Relay OP_RETURN TxOut as standard transaction type

Also fix decoderawtransaction to not show reqSigs or addresses for
nulldata txouts. (Previous version also left reqSigs uninitialized
mistakenly)
Show short scriptPubKeys correctly

Previously bitcoin-qt's -debug transaction info was showing CTxOut([error])

It is valid for a scriptPubKey to be any size, for example simply
OP_RETURN is valid and can be used to destroy a TXOUT to mining fees.
Give peer time-adjustment data an own lock

Instead of relying on cs_main (defined in a different module) to
prevent concurrent access to it.
Do not use the redundant BestInvalidWork record in the block database.

As block index entries have a flag for marking invalid blocks, the
'best invalid work' information can be derived from there. In addition,
remove the global from main.h
Make some globals in main non-public.

This means they are declared static, and their extern definition in
main.h is removed. Also moved CBlockIndexWorkComparator to the .cpp
file.
Merge pull request #3114

a616206 Give peer time-adjustment data an own lock (Pieter Wuille)
Merge pull request #3128

0056095 Show short scriptPubKeys correctly (Peter Todd)
22de68d Relay OP_RETURN TxOut as standard transaction type (Peter Todd)

Signed-off-by: Gavin Andresen <gavinandresen@gmail.com>
Merge pull request #3087

caca6aa Make some globals in main non-public. (Pieter Wuille)
85eb2ce Do not use the redundant BestInvalidWork record in the block database. (Pieter Wuille)