Functions that return storage prefixes should return `&[u8]`

[mooori]

The functions that return storage prefixes (e.g. Pausable::pa_storage_key) should return &[u8] instead of &'static [u8]. This was pointed out by @birchmd here.

It is a trivial change for all plugins except AccessControllable as described here.

Steps to resolve this issue:

• Refactor the default implementation of AccessControllable such that acl_storage_prefix can take parameter &self.
• Change the corresponding functions of all plugins to return &[u8].