localbuf: Track pincount in BufferDesc as well

For AIO on temporary table buffers the AIO subsystem needs to be able to
ensure a pin on a buffer while AIO is going on, even if the IO issuing query
errors out. Tracking the buffer in LocalRefCount does not work, as it would
cause CheckForLocalBufferLeaks() to assert out.

Instead, also track the refcount in BufferDesc.state, not just
LocalRefCount. This also makes local buffers behave a bit more akin to shared
buffers.

Note that we still don't need locking, AIO completion callbacks for local
buffers are executed in the issuing session (i.e. nobody else has access to
the BufferDesc).

Reviewed-by: Noah Misch <noah@leadboat.com>
Discussion: https://postgr.es/m/uvrtrknj4kdytuboidbhwclo4gxhswwcpgadptsjvjqcluzmah%40brqs62irg4dt

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