Remove auxpow field from blkindex.dat storage.

Disable storing of the auxpow field in blkindex.dat.  Instead, only
store there if the block has auxpow.  Whenever the auxpow itself is
needed, load it directly from the blockchain instead.
Rewrite blkindex.dat after compaction.

After removing the auxpow from the blkindex.dat database, rewrite the DB
file in order to remove "reserved" empty pages from it entirely.
Disable 'getheaders' network message.

Disable the 'getheaders' network message, since it is never actually
used and thus also not well tested.
Force fReadTransactions=true for reading blocks.

Remove fReadTransactions as optional argument in CBlock::ReadFromDisk
for CBlockIndex* arguments, since this is never actually used in the
code.  This allows to get rid of the CBlockIndex::GetBlockHeader method.
Completely remove auxpow field from CBlockIndex.

Completely remove the auxpow field from CBlockIndex, since it is
actually not necessary anywhere.
Allow setting of stream version in the DB.

Allow setting of the nVersion parameter used for stream serialisation in
the database.
Make use of DB serialisation version.
Don't include nVersion field in CBlockIndex disk serialisation.

Skip the nVersion field in the serialisation of each CBlockIndex object
to disk (the block header nVersion is still there).  It is unnecessary,
since we use the global DB's verion field.
Remove unused skip field and don't overwrite version in CDB::Rewrite.

In CDB::Rewrite, just do a literal rewrite.  Don't skip fields and don't
overwrite the version.
Fix setting of stream versions for LoadBlockIndex.
Merge pull request #38 from domob1812/remove-auxpos

Remove auxpow from CBlockIndex