Fix tracing of BackendKeyData and CancelRequest

Fix tracing of BackendKeyData and CancelRequest

BackendKeyData length was increased from 4 bytes to a variable-length
length (up to 256 bytes) in a460251f0a. However, pqTrace still traces
it as a 4 bytes key, leading to a "mismatched message length" warning
message. The same issue impacts the tracing of CancelRequest.

This patch fixes the issue by using pqTraceOutputNchar instead of
pqTraceOutputInt32 in both cases.

Author: Anthonin Bonnefoy <anthonin.bonnefoy@datadoghq.com>
Discussion: https://www.postgresql.org/message-id/CAO6_Xqo6gTv9=76H=k2qDRFU+KHuBiY2S=bQynEr6J8gS7L6xA@mail.gmail.com
Backpatch-through: 18