Log a warning if PG startup takes too long

[nyurik]

Per discussion in #804, Martin should warn users if PostgreSQL discovery step is taking too long.

For each PostgreSQL connection, e.g. in PgConfig::resolve, print a warning if table/function instantiation takes longer than 5 (?) seconds. This could probably be done using this method waiting for the instantiate_tables function. The instantiate_functions could also use some sort of a warning in case we implement retrieving a test tile, but it probably won't have the same message.

So, if instantiate_tables takes over 5 seconds, print something like this:

WARN: PostgreSQL table discovery is taking too long. Make sure your table geo columns have a GIS index, or use --disabling-bounds.