Create object in backend for buckets to affirm their existence and to
store meta data. Support S3 bucket listing. Generalise bucket logic to
'container' where the protocol should not be known.

Change log
* Disk
    - append to bucket list instead of overwriting
    - parse bucket name from object path
    - store list of objects in each bucket in a unique file
    - store list of buckets in a file
    - get_object_list function
* use bucket + object path in unit tests
* test Fifo::get_bucket_object_list
* bucket listing in stream, storage and disk implementation
* list buckets using Phobos
* test for Phobos bucket listing
* function to format ListBuckets response
* bucket list and object list request identification
* handle bucket list request
* list buckets integration test
* https confict response if bucket already exists for create_container
* bucket list unit tests ignore buckets created by previous test runs
* function to reset Phobos descriptor
* function to reset Fifo descriptors and storage backend
* make sure to deallocate objid if it is allocated before reallocating
* function to check if object exists in the Phobos store
* function to check if object exists in Disk store
* test object_exists functions
* create container if object is PUT to a not yet existing one
* create container function in PutRequestHandler
* remove bucket logic from Disk
* remove bucket tests from Phobos_file and Fifo tests
* remove bucket logic from Stream and Storage as this is handled elsewhere
* handle bucket logic in Protocol Formatter
* rename bucket to container where appropriate
* support create bucket request
* test create bucket request
* function to check for container list request
* use sane bucket identifier
* function to extract container name
* use try-catch in create_container function
* test that creating an existing container fails
* update documentation
* flush log messages to logfile immediately