client: read /etc/resolv.conf less often

[hannesm]

When discussing #291 with @reynir, we figured that /etc/resolv.conf may be read rather often and its content being checksummed (using Digest.string --> MD5).

We thought about various solutions, all increasing the complexity:

• remember a timestamp of the last read, and only re-read if at least 2 seconds have passed
• remember the mtime of /etc/resolv.conf and only re-read if the mtime changed
  • this is brittle if /etc/resolv.conf is a symlink, thus we need to preserve its and linked-to-files paths and mtimes

It is not clear whether it is worth the effort and complexity, so we'll leave as is -- and if the current code turns out to be a bottleneck, we'll revise it.