always use assertutil for assertions

[zooko]

Debugging #1742, we have the following report from a customer:

  File "/root/allmydata-tahoe-1.10.0/src/allmydata/mutable/retrieve.py", line 299, in _setup_download
    assert len(self.remaining_sharemap) >= k
exceptions.AssertionError: 

It would be helpful if we had instead received this from the customer:

  File "/root/allmydata-tahoe-1.10.0/src/allmydata/mutable/retrieve.py", line 299, in _setup_download
    _assert(len(self.remaining_sharemap) >= k, lenr=len(self.remaining_sharemap), k=k)
  File "/root/allmydata-tahoe-1.10.0/src/allmydata/util/assertutil.py", line 24, in _assert
    raise AssertionError, "".join(msgbuf)
AssertionError: lenr: 0 <type 'int'>, 'k': 1 <type 'int'>

Our wiki:CodingStandards doc says to use assertutil. To close this ticket, find all uses of assert in our codebase and change them to assertutil.

[daira]

This would also fix #1944.

[daira]

This would also fix #1085.