* Added support for account signing keys. When account signing keys change
the validity of the client JWT and token imports need to be checked as well
as it is possible for the signing key used to sign the user or import
token to have been removed from the source account.
Changed account lookup and validation failures to be more understandable by users.
Changed limits to be -1 for unlimited to match jwt pkg.
The limits changed exposed problems with options holding real objects causing issues with reload tests under race mode.
Longer term this code should be reworked such that options only hold config data, not real structs, etc.
Signed-off-by: Derek Collison <derek@nats.io>
