Fix cloud backend on Windows

[daira]

See ​https://ci.appveyor.com/project/tahoe-lafs/tahoe-lafs/build/1.0.199

There appears to be a problem with paths, e.g.:

[ERROR]
Traceback (most recent call last):
  File "c:\projects\tahoe-lafs\src\allmydata\storage\backends\cloud\cloud_common.py", line 369, in _handle_error
    raise first_err.__class__, first_err, first_tb
allmydata.storage.backends.cloud.cloud_common.CloudError: ("try 1 failed: PUT object ('shares/wn/wn663zg7yip4ummfral6lykgtu/9',) {}", 123, 'The filename, directory name, or volume label syntax is incorrect')
allmydata.test.test_system.SystemWithCloudBackendAndMockContainer.test_upload_and_download_random_key

and a problem with file open modes:

[ERROR]
Traceback (most recent call last):
  File "c:\projects\tahoe-lafs\src\allmydata\test\test_system.py", line 479, in _corrupt_mutable_share
    share.pwrite(MutableDiskShare.DATA_OFFSET, final_share)
  File "c:\projects\tahoe-lafs\src\allmydata\scripts\debug.py", line 100, in pwrite
    self.write_to_chunk(chunknum, write_offset, data[data_offset : data_offset + write_length])
  File "c:\projects\tahoe-lafs\src\allmydata\scripts\debug.py", line 107, in write_to_chunk
    f = open(self._get_chunk_filename(chunknum), "rw+b")
exceptions.ValueError: Invalid mode ('rw+b')
allmydata.test.test_system.SystemWithDiskBackend.test_mutable

[daira]

I should open a new pull request for the .4 branch.

[exarkun]

Moving open issues out of closed milestones.

[exarkun]

The established line of development on the "cloud backend" branch has been abandoned. This ticket is being closed as part of a batch-ticket cleanup for "cloud backend"-related tickets.

If this is a bug, it is probably genuinely no longer relevant. The "cloud backend" branch is too large and unwieldy to ever be merged into the main line of development (particularly now that the Python 3 porting effort is significantly underway).

If this is a feature, it may be relevant to some future efforts - if they are sufficiently similar to the "cloud backend" effort - but I am still closing it because there are no immediate plans for a new development effort in such a direction.

Tickets related to the "leasedb" are included in this set because the "leasedb" code is in the "cloud backend" branch and fairly well intertwined with the "cloud backend". If there is interest in lease implementation change at some future time then that effort will essentially have to be restarted as well.