Context

While reading both the code and documentation to understand the inbox I noticed a few flaws:

consume_n_messages transforms a int32 to int to int64. I save a transformation by removing the int transformation.
Use fail_when in add_external_messages
Indent correctly the docstrings

Manually testing the MR

There is no semantically changes expect for the consume_n_messages which is covered in unit tests.

n/a Document the interface of any function added or modified (see the coding guidelines)
n/a Document any change to the user interface, including configuration parameters (see node configuration)
Provide automatic testing (see the testing guide).
n/a For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
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