Restore suggestion responsiveness

[nilmerg]

Since #1173 suggestions are delayed until custom variables were identified. This leads to a noticeable delay of column suggestions. Previously they were prioritized and as long as there were 50 of them identified, custom variables were not even fetched.

I propose to still prioritize columns over custom variables. Though, we can keep the new Best Suggestions feature.

Reason for this is, a best suggestion is technically an exact match. An exact match is very easy to identify for a database. So, let's query the custom variables still first, but only look for exact matches. (i.e. remove all trailing and leading wildcards from the search term but still make sure there's no wildcard left)

Then again, columns next. They are allowed to consume the limit of 50. So that custom variables that don't exactly match only appear if there are not enough columns.