Previously, NewManifest was asynchronous so subsequent code which tried
to use the returned manifest could error as the manifest was not yet
persisted.
The previous attempt to use --ethapi as a fallback if --ens-api is not
set does not work because --ens-api has a default value, and also
setting --ens-api to "" is the suggested way to disable ENS lookups.
Signed-off-by: Lewis Marshall <lewis@lmars.net>