Fixed #513 -- Properly truncate available names when file_overwrite = True

[jschneier]

No description provided.

[jschneier]

Need to fix for other backends but what do we think about this approach?

[jschneier]

@nitely is AzureStorage affected by this? My reading is yes but there is some special _AZURE_MAX_LEN indirection in there and just want to confirm.

[nitely]

@jschneier I'm afraid it is. _AZURE_MAX_LEN just makes sure the name doesn't exceed azure's name length limit. Without it, it would raise later when uploading the file (hopefully, but I didn't try it).