grpc: handle invalid service configs by applying the default, if applicable

[easwars]

If the name resolver returns an invalid service config, and if there is no current service config on the channel, this PR attempts to apply the default service config, if one is available.

This aligns grpc-go with https://github.com/grpc/proposal/blob/master/A21-service-config-error-handling.md, specifically #4.2

This PR also makes the following changes:

• handle nil receiver in ccBalancerWrapper
• remove cc.balancerWrapper != nil checks before calling methods on the balancerWrapper
• use cc.sc != nil as the condition representing presence of current service config instead of cc.balancerWrapper != nil

RELEASE NOTES:

• handle invalid service configs by applying the default, if applicable