Added a module docstring as a usage() string.  Also add --help/-h.

Rewrote two of the regexps as verbose re's for readability <wink>.
Use named groups.

Use True/False where appropriate.

Txn: Added a `voters' attribute which is a list of clients seen doing
a vote.  This is cleared on abort or finish.  Thus if we see a
transaction with a non-empty voters list, we know that that
transaction was blocked and it was the first voter that grabbed the
lock.  This client (as an address) is displayed in the report.

call_*(): Pass the client address into the call, as grep'd out of the
zrpc-conn part of the line.  Only call_tpc_vote() really cares about
this.

Most controversial: process the entire file, line by line.
Alternatives, if necessary:
    - go back to the original progressive skip backwards approach
    - add an incremental reader (recording file positions)
    - use as a filter