This commit fixed 2 issues wrt Backblaze B2 authorization:
* every thread may call b2_authorize_account at the same time when there
are 401 errors
* if B2 has a login outage, then all threads will call b2_authorize_account
repeatedly without delay
A simple solution is to limit one b2_authorize_account call to once every
30 second regardless of how many threads there are. If the call to
b2_authorize_account is not allowed, the random exponential backoff will
be performed.
* All APIs include UploadFile are done via the call() function
* New retry mechanism limiting the maximum backoff each time to 1 minute
* Add an env var DUPLICACY_B2_RETRIES to specify the number of retries
* Handle special/unicode characters in repositor ids
* Allow a directory in a bucket to be used as the storage destination
