Fix worker_spi when launching workers without shared_preload_libraries

Fix worker_spi when launching workers without shared_preload_libraries

Currently, the database name to connect is initialized only when the
module is loaded with shared_preload_libraries, causing any call of
worker_spi_launch() to fail if the library is not loaded for a dynamic
bgworker launch.  Rather than making the GUC defining the database to
connect to a PGC_POSTMASTER, this commit switches worker_spi.database to
PGC_SIGHUP, loaded even if the module's library is loaded dynamically
for a worker.

We have been discussing about the integration of more advanced tests in
this module, with and without shared_preload_libraries set, so this
eases a bit the work planned in this area.

No backpatch is done as, while this is a bug, it changes the definition
of worker_spi.database.

Author: Masahiro Ikeda
Reviewed-by: Bharath Rupireddy
Discussion: https://postgr.es/m/d30d3ea7d21cb7c9e1e3cc47e301f1b6@oss.nttdata.com