Mark object store bucket as full and move new files into new bucket

[MorrisJobke]

Problem

• the vendors of object stores give out a recommendation the maximum number of objects per bucket
• Nextcloud should respect those limits and if a bucket is close to this value it should not put more files into it
• new files should be put into a newly created bucket
• multibucket setups should be able to be marked as "extensible" - so there is a default bucket count and as it's capacity of a bucket is reached then a new bucket is created

Things to keep in mind

• we should look into storing the bucket a file is located in instead of calculate it on the fly:
  • instead of having this implicitly (calculating the hash and have a formula in which bucket this hash goes) we should store the result in the database so we can move different files that would be in the same bucket can also be moved based on the amount of files in the bucket
• what about a migration? this would allow to rebalance buckets that are already over this limit and misbehave for that reason

@rullzer @kesselb @icewind1991 Feedback and ideas are welcome.

[icewind1991]

My idea would be to save "ranges" of fileids in a table for each multibucket storage, to split fileids into further buckets

With a table looking something like

storage bucket	start	end	postfix
"bucket_1"	1	100	""
"bucket_1"	101	null	"a"
"bucket_2"	1	null	""

meaning that files storaged within the multibucket storage "bucket_1" with fileids between 1 and 100 will be stored in bucket "bucket_1", with fileids after 101 in "bucket_1a".

Since the fileids are already spread over multibucket this means that there is an overlap between the ranges for different "storage buckets", and the bucket "bucket_1" will have less files in them then their range might suggest.

When the storage needs to call the object storage, it would find the range the fileid belongs in to find the final bucket name. When a new object is written, the number of objects the the current, final range can be queries from the filecache and if the configured limit has been exceeded, a new range is created.