Proxy primary redirect timeout

[benbjohnson]

This pull request changes the HTTP proxy so that it will wait until it has primary information before attempting to redirect. It will wait up to 5 seconds by default but this can be changed in the config.

Fixes https://github.com/superfly/litefs/issues/406

Config

To change the primary redirect timeout, you can specify it in the proxy section of the config:

proxy:
  addr: ":8080"
  target: "localhost:8081"
  db: "my.db"
  primary-redirect-timeout: "10s"