config: delayed privilege alert doesn't stuck now

The declarative configuration has the `credentials` section that
describes users and their privileges. It is OK to have privileges for a
space/function/sequence that does not exist. Such a privilege will lead
to an alert that states that the privilege will be granted, when the
object is created.

The problem that is fixed by this commit is that such an alert was not
dropped, when the object is created and the relevant privileges are
granted.

There are several ways to solve the problem. Let's look on them.

1. When a privilege is granted, drop an alert if any.
2. After the config-database privilege synchronization, revisit alerts
   to drop all obsolete ones.
3. Drop all the alerts regarding missed privileges before the
   config-database privilege synchronization and issue actual alerts
   afterwards.

The first way is the simplest, but it doesn't cover one specific
scenario: an object rename.

Let's assume that the object T has privileges declared in the
configuration and the object doesn't exist. There is an alert regarding
it. Now, object S is renamed to T. Let's assume that S had some or all
the privileges needed for T according to the configuration.

In the given scenario, we don't need to grant some or all of the
privileges and, so, the first solution doesn't work. We don't reach the
code that grants the privileges and, so, dropping alerts at this point
has no effect.

The second and the third solutions are similar and mainly differs in how
complicated the code is. The third one is implemented here with idea of
simplifying the code.

The internal `aboard` module has the following changes.

1. The `aboard` module now ignores underscored fields of an alert on its
   serialization to allow a caller to store a machine-readable
   information in them.
2. The new method `:drop_if()` is added to perform a conditional alert
   drop.

Several unit test cases are updated, because now we always need
initialized `config._aboard` for testing of the credentials applier.

Fixes #9574

NO_DOC=bugfix