Improve readability of the output of psql's \timing command.

Improve readability of the output of psql's \timing command.

In addition to the existing decimal-milliseconds output value,
display the same value in mm:ss.fff format if it exceeds one second.
Tack on hours and even days fields if the interval is large enough.
This avoids needing mental arithmetic to convert the values into
customary time units.

Corey Huinker, reviewed by Gerdan Santos; bikeshedding by many

Discussion: <CADkLM=dbC4R8sbbuFXQVBFWoJGQkTEW8RWnC0PbW9nZsovZpJQ@mail.gmail.com>