BlockHoover and cmd_getblocks tweaks.

BlockHoover:
  push: new method (adds to self.queue & self.qset).
  go:
    rather than fetching a new live connection, use the one that reported a greater height.
  cmd_getblocks:
    send inv response directly (i.e., without filtering). [seems to be required by core client].
new args for filtering out 'block' packets & rendering binary strings in hex.