New Azure SDK Version 0.15.0 (memory leak fix)

Follow up on cases:
Ask for settings recommendation for azureblob
and
Uploading a file leaks a timer and context #253
Microsoft released a preview version of SDK 0.15.0 with fixes to this problem:
Azure Preview SDK Change Log

@ncw, can you build up a new version of rclone based on a new version of SDK?

Best regards,
Raymond

I've made a beta for you to try

v1.59.0-beta.6100.c726ae3af.fix-azureblob-leak on branch fix-azureblob-leak (uploaded in 15-30 mins)

Let me know how it goes!