Only update last seen time when the node is actually being useful. #4451

More useful tracking of useful peers (ones that actually serve useable blocks and txs, rather than version messages, inv messages, getdata messages).

Gotta be honest, I don't fully understand what this code does - but somehow this change seems to intuitively make more sense than the current code.

Automatic sanity-testing: FAILED BUILD/TEST, see http://jenkins.bluematt.me/pull-tester/p4451_ba6166ee916e015f39429fe36b24242861c971a4/ for binaries and test log.

This could happen for one of several reasons:

1. It chanages paths in makefile.linux-mingw or otherwise changes build scripts in a way that made them incompatible with the automated testing scripts (please tweak those patches in qa/pull-tester)
2. It adds/modifies tests which test network rules (thanks for doing that), which conflicts with a patch applied at test time
3. It does not build on either Linux i386 or Win32 (via MinGW cross compile)
4. The test suite fails on either Linux i386 or Win32
5. The block test-cases failed (lookup the first bNN identifier which failed in https://github.com/TheBlueMatt/test-scripts/blob/master/FullBlockTestGenerator.java)

If you believe this to be in error, please ping BlueMatt on freenode or TheBlueMatt here.

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.