Use read streams in CREATE DATABASE when STRATEGY=WAL_LOG.

Use read streams in CREATE DATABASE when STRATEGY=WAL_LOG.

While this doesn't significantly change runtime now, it arranges for
STRATEGY=WAL_LOG to benefit automatically from future optimizations to
the read_stream subsystem.  For large tables in the template database,
this does read 16x as many bytes per system call.  Platforms with high
per-call overhead, if any, may see an immediate benefit.

Nazir Bilal Yavuz

Discussion: https://postgr.es/m/CAN55FZ0JKL6vk1xQp6rfOXiNFV1u1H0tJDPPGHWoiO3ea2Wc=A@mail.gmail.com