Closely track mempool byte total. Add "getmempoolinfo" RPC.

Goal:  Gain live insight into the mempool.  Groundwork for future work
that caps mempool size.
build: fix automake warnings about the use of INCLUDES

While we're at it, reduce the use of LIBS as well. This makes dependencies
explicit.

Fixes building with (the not-yet-merged) libsecp256k1 as well.

Github-Pull: #4689
Rebased-By: Wladimir J. van der laan <laanwj@gmail.com>
Rebased-From: 909b347 c0e5dda
Merge pull request #4638

6f2c26a Closely track mempool byte total. Add "getmempoolinfo" RPC. (Jeff Garzik)
Add travis CI builds.
Tidy up pkg.m4 into src/m4. Add minimal bitcoin.pc.in.
Fix CPPFLAGS for libbitcoin_cli