lib-storage: Add mailbox_attribute_unregister_internal[s]() This allows plugins to unregister internal attributes when they're unloaded.

spelling fixes

lib-storage: remove unused internal_attribute argument from attribute_{get,set} This removes an argument added in 9f37ef2a9192e7d47e3d7ac959080fd01120f2e9 but obsoleted (but not removed) in d1147c225f64596fc5eeb1cb2aab31b3c57d5215. Not only does this change simplify the codebase, it actually brings the API closer to what it was back in 2.2.

lib-storage: document mailbox and server attribute handling

lib-storage: Change mailbox_attribute_get* API lib-storage/mailbox-attribute.h: mailbox_attribute_get() and mailbox_attribute_get_stream() now has struct mailbox_transaction_context as their first parameter. This isn't good for performance, because opening a transaction requires opening mailbox indexes first. But reading attributes wouldn't actually require opening the indexes, so this is just wasteful.

lib-storage: Added mailbox_attribute_register_internals() Just for making it easier to register multiple attributes.

lib-storage: Added MAIL_ATTRIBUTE_INTERNAL_FLAG_CHILDREN This also adds the key to get() and set() functions, so they can know exactly what key is wanted to be accessed.

mail-storage: Added registration API for builtin mailbox attributes.

lib-storage: Updated MAILBOX_ATTRIBUTE_KEY_IS_USER_ACCESSIBLE() to not allow private server attributes.

lib-storage: Fixed setting/getting server metadata. Also added a MAILBOX_ATTRIBUTE_KEY_IS_USER_ACCESSIBLE() macro to make it a bit easier to check if a key should be accessible to a user or not.

mail-storage: Moved mailbox attributes API to separate module.