Skip Conduit call log writes in read-only mode, allowing "conduit.ping" to run

Summary: Ref T10769. See PHI8. We have an unconditional logging write which we can skip in read-only mode.

Test Plan:

• Put Phabricator in read-only mode with cluster.read-only.
• Called conduit.ping via web UI.
  • Before: write-on-read-only exception.
  • After: good result.

Reviewers: chad, amckinley

Reviewed By: chad

Maniphest Tasks: T10769

Differential Revision: https://secure.phabricator.com/D18233