Change the gossip pull engine to memoize the timeout configuration at
construction instead of constantly retrieving it at runtime from viper.

This resolves a race where TestUpdateRootsFromConfigBlock modifies the
viper configuration causing a concurrent map modification race.

Also add missing error assertions in TestUpdateRootsFromConfigBlock and
update the formatting to be a bit more idiomatic.

Change-Id: I094b7426e494bdcff00e156f9e804c58aceb9f06
Signed-off-by: Matthew Sykes <sykesmat@us.ibm.com>