Add flush status check in SendPacket

[crodriguezvega]

Pre-requisite:

• ics04: Channel Upgrade Packet Flushing Logic ibc#982

We already check if the channel state is OPEN, so we need to add a check to allow sending packets only if flush status is NOTINFLUSH as well.

[chatton]

PR is merged, but we can keep this issue open in case changes happen in the spec and create a follow up PR.

[crodriguezvega]

Spec changes are merged now, so I will close the issue.

Closed by #3912