Bridges start matrix-synapse.service as a dependency, but
Synapse is sometimes slow to start, while bridges are quick to
hit it and die (if unavailable).

They'll auto-restart later, but .. this still breaks `--tags=start`,
which doesn't wait long enough for such a restart to happen.

This attempts to slow down bridge startup enough to ensure Synapse
is up and no failures happen at all.

Fixes #171 (Github Issue).