storage: fix storage_update API

[DingMeng-Siemens]

The storage_update need to change since the _storage_write is removed.
to solve issue #366
cc @chombourger @bovi

[chombourger]

I am adding ImageFile.new() with #369
if used here, it will let us storage update a file which is either local or remote

[DingMeng-Siemens]

OK I will change.

[chombourger]

thank you. I have merged #369. you should be able to rebase your work

[chombourger]

with #369, prepare() can be told the compression format to use
(file.prepare(imgsize, compression=CONSTS.IMAGE.RAW.value))

[DingMeng-Siemens]

OK I will change.

[DingMeng-Siemens]

Refined the code.
I also removed _self.writer.start() from storage_update() in main.py, because this will create an unmanaged writing thread and that will cause strange behavior since we already call storage_open() to create writing thread in storage_update() in client.py.
cc @chombourger Please check.

[chombourger]

this portion of the comment is not relevant (bmap files are only used with storage write)

[chombourger]

Changes look good. just one comment needs to be updated

[DingMeng-Siemens]

changed

[bovi]

@DingMeng-Siemens great work!

@chombourger thanks for merging