Lazily interpolate strings when logging

 - Fix all logging calls to use lazy string interpolation to avoid
   unnecessarily casting megabytes of data to strings.
 - Convert congress.policy.runtime.iterstr to a class that lazily
   joins the iterable.
 - Very minor style tweaks in related blocks of code (except X as Y
   syntax, remove unnecessary parentheses, etc).

Change-Id: If2cd68a997f59d706295c0a2687e5f315c41c5e5
Merge "Lazily interpolate strings when logging"