structure view performance degrades with large column count

while the content view for large tables performs better than any other postgres app, the structure view performance degrades noticeably for tables with more than 40 columns. Adding additional columns takes longer for each new column. To reproduce just create a new table and hit the `+ Column` button a bunch of times. After `column50` it takes 3 seconds on my macbook air for the new column to appear, after `column70` the app starts to beachball for a while.

Reproduce the slowdown in the structure view by creating a table and repeatedly using the `+ Column` button. Observe the time to add columns around column50 and column70, then trace the structure-view column-add path. Done means adding columns no longer degrades noticeably or beachballs with large column counts.

Written by the indexing model from the issue text.