Allow multiple blob_dir

[jessopb]

A user said he would love to host more, but it's not easy for him to move to other/external hd.
This would allow users to host more content.

• user can add additional blob_dir folders, mark default (like steam)
• support freely moving blobs between blob_dir locations
• handle disconnection of a blob_dir folder (external HD?) - mark db entries temporarily missing?

[kodxana]

Add support to S3 buckets mayby :)

[shyba]

first version:

• specify new folder (first of the list in the conf file)
• if we have permissions and everything is good, use it
• otherwise, warn and fallback to next
• fails startup if all items fail

for now, moving blobs: stop sdk, move, start (we check on startup already)

Handling disconnection of a blob directory might be another issue as it can already happen if the current directory is an external drive. Not sure what to do here yet