Cleanup refactoring of coins/mempool #3199

This pull request should have no functional effects.

• The first commit removes the CBlockIndex dependency of CCoinsView, making it reason in terms of block hashes (which it was doing under the hood anyway).
• The second commit moves most CCoins/CCoinsView related logic to coins.{h,cpp}, and the rest to txmempool.{h,cpp}. This removes the need for the LookupFromTip loophole, as txmempool is just built on top of coins.

These should move to coins.cpp, too, I think:

main.cpp:const CTxOut &CCoinsViewCache::GetOutputFor(const CTxIn& input)
main.cpp:int64 CCoinsViewCache::GetValueIn(const CTransaction& tx)
main.cpp:bool CCoinsViewCache::HaveInputs(const CTransaction& tx)

@gavinandresen Nice catch, done.

Rebased.

ACK assuming pull-tester is happy.

Let's delay this until #2767 is merged.

Rebased on top of #2767.

Automatic sanity-testing: PASSED, see http://jenkins.bluematt.me/pull-tester/a0fa20a12b69717636050dd8742713b1d82ae524 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.

ACK; re-tested running -debug=mempool on the main network, with additional testing running a couple of -regtest peers (creating blocks, sending transactions back and forth, etc).