Use more privileged memory allocation for the plaintext Buffer in Decrypt

### Problem:
[https://github.com/aws/aws-encryption-sdk-javascript/blob/master/modules/decrypt-node/src/decrypt.ts](url)
```javascript
const plaintext: Buffer[] = []
let messageHeader: MessageHeader | false = false
stream
.once('MessageHeader', (header: MessageHeader) => {
messageHeader = header
})
.on('data', (chunk: Buffer) => plaintext.push(chunk))

// This will check both Uint8Array|Buffer
if (ciphertext instanceof Uint8Array) {
stream.end(ciphertext)
} else if (typeof ciphertext === 'string') {
stream.end(Buffer.from(ciphertext, encoding))
} else if (ciphertext.readable) {
ciphertext.pipe(stream)
} else {
throw new Error('Unsupported ciphertext format')
}

await finishedAsync(stream)
if (!messageHeader) throw new Error('Unknown format')

return {
plaintext: Buffer.concat(plaintext),
messageHeader,
}
```

The `Buffer.concat(plaintext)` will result in a new Buffer. However, the `const plaintext: Buffer[] = []` seem not zero-out where sensitive data might remain in memory until gc.

[//]: # (NOTE: If you believe this might be a security issue, please email aws-security@amazon.com instead of creating a GitHub issue. For more details, see the AWS Vulnerability Reporting Guide: https://aws.amazon.com/security/vulnerability-reporting/ )