Fix comparison tool by asking for blocks more aggressively
Re-enable BitcoindComparisonTool:

 * Use the latest version, with limited memory usage, and path to
   on-disk db (try mouting qa/tmp on a tmpfs)\
 * enable -debug=net
 * re-enable BitcoindComparisonTool in pull-tester
Make large-reorg tests optional in block-tester
pull-tester.py: Re-enable coverage msg, new args to run test script
Merge pull request #3164 from TheBlueMatt/master

Re-enable BitcoindComparisonTool on pull-tester