client: make use of shutdownLock on Read access

[hw-claudio]

c.shutdownLock protects access to c.shutdown,
but before this commit this is only used when calling Close(), thus only protecting against parallel user calls to Close(),

but it was not used when checking whether the channel is shutdown in the exported function IsClosed(), potentially returning an incorrect value.

It was also not used in send() and listen().

To address this, turn the c.shutdownLock Mutex into a RWMutex, and make use of it in all these cases, enabling use of the same RPCClient from multiple parallel goroutines.

Fixes: #782

[hashicorp-cla-app]

Thank you for your submission! We require that all contributors sign our Contributor License Agreement ("CLA") before we can accept the contribution. Read and sign the agreement

Learn more about why HashiCorp requires a CLA and what the CLA includes

---

Claudio Fontana seems not to be a GitHub user.
You need a GitHub account to be able to sign the CLA.
If you have already a GitHub account, please add the email address used for this commit to your account.

_{Have you signed the CLA already but the status is still pending? Recheck it.}

[hashicorp-cla-app]

Thank you for your submission! We require that all contributors sign our Contributor License Agreement ("CLA") before we can accept the contribution. Read and sign the agreement

Learn more about why HashiCorp requires a CLA and what the CLA includes

---

Claudio Fontana seems not to be a GitHub user.
You need a GitHub account to be able to sign the CLA.
If you have already a GitHub account, please add the email address used for this commit to your account.

_{Have you signed the CLA already but the status is still pending? Recheck it.}

[hw-claudio]

signed the agreement