Allowing to change the total or initial resources the semaphore has. After calling `set_resources()` the semaphore will look like as if it was created with the specified amount of resources when created.
Use the new method in `replica::database::revert_initial_system_read_concurrency_boost()` so it doesn't lead to strange semaphore diagnostics output. Currently the system semaphore has 90/100 count units when there are no reads against it, which has led to some confusion.
I also plan on using the new facility in enterprise.
Closes#11772
* github.com:scylladb/scylladb:
replica/database: revert initial boost to system semaphore with set_resources()
reader_concurrency_semaphore: add set_resources()
6b0afb968d