This commit marks the start/stop functions in util.py as private module
functions. A future PR will remove these entirely and move the
functionality directly into the BitcoinTestFramework class, but setting them as
private in this PR will prevent anyone from accidentally calling them
before that future PR is merged.
This commit reduces spammy logging by the test framework. It truncates
logging send/receive message in mininode to 500 characters. mininode
was previously logging the entire message sent received, which can be up
to 1MB for a full block.
