Fix pg_atomic_u64 initialization.

Fix pg_atomic_u64 initialization.

As Andres pointed out, pg_atomic_init_u64 must be used to initialize an
atomic variable, before it can be accessed with the actual atomic ops.
Trying to use pg_atomic_write_u64 on an uninitialized variable leads to a
failure with the fallback implementation that uses a spinlock.

Discussion: https://www.postgresql.org/message-id/20170816191346.d3ke5tpshhco4bnd%40alap3.anarazel.de