Send 'mempool' P2P command at the start of each P2P session

[jgarzik]

to query remote node mempool contents.

This supercedes pull #1833, which was fat-finger merged, and then resultant damage undone.

See #1833 for relevant comments on this pull request.

[BitcoinPullTester]

Automatic sanity-testing: FAILED BUILD/TEST, see http://jenkins.bluematt.me/pull-tester/6c4ef621c8e1b65454cc90a9d7877c81813e2263 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
2. It does not build on either Linux i386 or Win32 (via MinGW cross compile)
3. The test suite fails on either Linux i386 or Win32
4. The block test-cases failed (lookup the first bNN identifier which failed in https://github.com/TheBlueMatt/test-scripts/blob/master/FullBlockTestGenerator.java)

[sipa]

@jgarzik Any idea why BitcoinPullTester complains?

[jgarzik]

Closing, for now. Need to think more about @gmaxwell 's comments (see #1833, the previous version of this pull req) with regards to the proposed expiration of old, unconfirmed transactions in the memory pool.

[luke-jr]

Also note this broke Bitcoin-Qt builds. Looks like we need that startup time variable @Diapolo suggested a while back...