Move CInv to protocol.[ch]pp

This commit does *not* and should not modify *any* code, it only moves
it from net.h and splits it across protocol.cpp and protocol.hpp.

Signed-off-by: Giel van Schijndel <me@mortis.eu>
Merge branch 'code-cleanup' of git://github.com/muggenhor/bitcoin
Merge commit 'e4dde849ae5544383703ef2d73592677e6c528ad' into namecoin_merge_v0.8
Namecoin: fix bnProofOfWorkLimit & nTotalBlocksEstimate