After #2285 is done, AccountStorageMode::Network should have been removed, and once that happens, we can simplify the NoteTag::with_account_target API further and use the full 32 bits of a note tag instead of at most 30 bits.

One difficulty is that RoutingParameters relies on NoteTag::MAX_ACCOUNT_TARGET_TAG_LENGTH to be 30 since its 5-bit format can only encode 2^5 = 32 values, and 31 is reserved to signal the absence of a note tag.

The potential solutions are:

• Allow up to 32 bits in RoutingParameters::note_tag_len and:
  • Reduce address interface from 11 to 10 bits and increase note tag from 5 to 6 bits.
    • This would waste quite a bit of available space for the note tag.
  • Move note tag length encoding to a separate byte, making more room. This means two bytes in total for address interface and note tag, instead of one as now.
    • This wastes even more space.
• Continue enforcing that at most 30 bits can be encoded in RoutingParameters::note_tag_len even though the NoteTag itself can support up to 32.

My preference is the latter option:

• 30 bits in a note tag already sufficiently reduce the set of potential target accounts.
• Network notes targeted at accounts are no longer identified by the note tag, so the requirement for having it be as precise as possible is no longer relevant.
• Leaves the address as small as it is.

So the tasks would be:

• Refactor NoteTag account targets to allow for up to 32 bits.
• Introduce a RoutingParameters::MAX_NOTE_TAG_LENGTH = 30 and use it to enforce max note tag len, instead of relying on NoteTag::MAX_ACCOUNT_TARGET_TAG_LENGTH which would be 32 at that point.

Context: #2219 (comment)