Limit CNode::mapAskFor

Tighten resource constraints on CNode.

Rebased-From: d4168c8
Rebased-By: Wladimir J. van der Laan <laanwj@gmail.com>
From Bitcoin d03093: Limit CNode::mapAskFor by constraining it to MAX_INV_SZ which is hardcoded to
50,000 entries. Tested by temporarily constraining MAPASKFOR_MAX_SZ to 1 entry and observing return behavior in AskFor(const CInv& inv). Passed all Boost tests.
Merge pull request #713 from michilumin/1.8.1-dev-limit-mapaskfor

1.8.1 dev limit mapaskfor