Tags are a useful mechanism that could be used outside of alternator
namespace. My motivation to move tags_extension and other utilities to
db/tags/ was that I wanted to use them to mark "synchronous mode" views.
I have extracted `get_tags_of_table`, `find_tag` and `update_tags`
method to db/tags/utils.cc and moved alternator/tags_extension.hh to
db/tags/.
The signature of `get_tags_of_table` was changed from `const
std::map<sstring, sstring>&` to `const std::map<sstring, sstring>*`
Original behavior of this function was to throw an
`alternator::api_error` exception. This was undesirable, as it
introduced a dependency on the alternator module. I chose to change it
to return a potentially null value, and added a wrapper function to the
alternator module - `get_tags_of_table_or_throw` to keep the previous
throwing behavior.
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