Smart (floating) fees for the client

This is a much more conservative pull request than my previous smart-fees request.

It only tries to solve the problem:  what fee (or priority) is needed to get into a block right now?

It adds two new RPC calls: estimatefee and estimatepriority.

Test plan:

Restart bitcoind with this patch. Then run:

bitcoin-cli -rpcwait estimatefee       : EXPECT: -1  (not enough data to estimate)
bitcoin-cli -rpcwait estimatepriority   : EXPECT: -1  (not enough data)

Leave it running an hour or two, and repeat. EXPECT: reasonable fee/priority estimate.

Test different medians; EXPECT: estimatefee 0.1 is less than or equal to estimatefee 0.5.  Same for priority.

Test saving/restoring mempool/estimation state:  shutdown bitcoind, and restart.  EXPECT: same estimates as before.

Test Bitcoin-Qt fee/priority estimation:

Run with a wallet that has a combination of big/old and small/new unspent transaction inputs.
Turn on coin control.
Test send:  EXPECT: coin control values Do the Right Thing (fee turns red if not enough fee is included, preiority turns red if not enough priority to qualify, etc).