Add helper for checking if a connection exists and is open

[DimitrisJim]

Summary

There's many instances of code that requires a connection to exist and be open:

1. During AcknowledgePacket

2. During RecvPacket

3. During ChanOpenTry

4. During ChanOpenAck

5. During ChanOpenConfirm

6. During ChanCloseInit

7. During ChanCloseConfirm

Additionally, it seems it will also be required in validateProposedUpgradeFields as added at some point in #3451 where it was initially conceived.

In most cases, the connectionEnd that is retrieved using GetConnection is required at some later
step in the function, as such, if a new function is to be added, it should return the open connection end if it exists.

Proposal

Add a new function that encapsulates both these steps; checks for existence and if the connection's state is OPEN, Bikeshed the naming.

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For Admin Use

• Not duplicate issue
• Appropriate labels applied
• Appropriate contributors tagged/assigned

[crodriguezvega]

Let's create an IsOpen function in the Connectionend type that does the open check:

if !connection.IsOpen() {
  return errorsmod.Wrapf(connectiontypes.ErrInvalidConnectionState, "expected %s, got %s", connectiontypes.Open, connectionEnd.State)
}

func (connection ConnectionEnd) IsOpen() bool {
  return connection.State == OPEN
}

[damiannolan]

Closing based on the comment in #5691 from @crodriguezvega. I think the general consensus has been: explicit is better