#1931: fix that not all placeholders were supported in connection target filtering

[thjaeckle]

Added support for placeholders in Connection "targets":

• feature:id
• entity:id
• entity:namespace
• entity:name
• thing:id
• thing:namespace
• thing:name

Resolves: #1931

[thjaeckle]

@alstanchev @kalinkostashki Could you please have a look?
Some "inconsistency" I noticed (I expected that to work, but it didn't) when trying to formulate e.g. that I only want to be notified about "feature messages" via:

_/_/things/live/messages?filter=exists(feature:id)

[alstanchev]

LGTM 👍