This PR hits two goals for "object storage" effort
1. Sstables loader "knows" that sstables components are stored in a Linux directory and uses utils/lister to access it. This is not going to work with sstables over object storage, the loader should be abstracted from the underlying storage.
2. Currently class keyspace and class column_family carry "datadir" and "all_datadirs" on board which are path on local filesystem where sstable files are stored (those usually started with /var/lib/scylla/data). The paths include subsdirs like "snapshots", "staging", etc. This is not going to look nice for obejct storage, the /var/lib/ prefix is excessive and meaningless in this case. Instead, ks and cf should know their "location" and some other component should know the directory where in which the files are stored.
Said that, this PR prepares distributed_loader and sstables_directly to stop using Linux paths explicitly by making both call sstables_manager to list and open sstables object. After it will be possible to teach manager to list sstables from object storage. Also this opens the way to removing paths from keyspace and column_family classes and replacing those with relative "location"s.
Closes#12128
* github.com:scylladb/scylladb:
sstable_directory: Get components lister from manager
sstable_directory: Extract directory lister
sstable_directory: Remove sstable creation callback
sstable_directory: Call manager to make sstables
sstable_directory: Keep error handler generator
sstable_directory: Keep schema_ptr
sstable_directory: Use directory semaphore from manager
sstable_directory: Keep reference on manager
tests: Use sstables creation helper in some cases
sstables_manager: Keep directory semaphore reference
sstables, code: Wrap directory semaphore with concurrency
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