This commit refactors base.regulator into the 2.4 interfaces and adds a
filesystem configuration option `maxthreads` to configure the regulator.

By default `maxthreads` is set to 100. This means the FS driver is
limited to 100 concurrent blocking file operations. Any subsequent
operations will block in Go until previous filesystem operations
complete.

This ensures that the registry can never open thousands of simultaneous
threads from os filesystem operations.

Note that `maxthreads` can never be less than 25.

Add test case covering parsable string maxthreads

Signed-off-by: Tony Holdstock-Brown <tony@docker.com>