[Tagging] Opening hours too long for OSM

[Tobias Knerr]

> Am 12.10.2018 um 01:27 schrieb Simon Poole:
>> We have a number of keys for which the values can easily exceed 255
>> chars besides opening_hours, lane destinations and conditional
>> restrictions are good candidates. Not to mention changeset tags. With
>> other words it is a general problem which should be tackled with a
>> general solution.

I agree that this problem calls for a general solution, as it's not
specific to opening hours. And yes, an extension syntax seems like a
good choice for now. While it's going to be entered manually at first,
it would allow transparent editor support, and possibly an automatic
conversion if any future API revision should support longer values natively.

We need to make sure that this can be implemented in a key-agnostic
manner, though, i.e. by simply concatenating the values. This means: No
key-specific exceptions such as omitting semicolons in an opening hours
string.

I don't have any strong opinions on the syntax, but it should be
something that doesn't clash with existing uses. Tobias' suggestion to
use _2 suffixes would overlap with things like name_2 that are currently
used in the database, so I'd prefer using something else. Maybe double
underscores, or a character such as # that's not typically used in keys
at all?