injection_points: Add initialization of shmem state when loading module

injection_points: Add initialization of shmem state when loading module

This commits adds callbacks to initialize the shared memory state of the
module when loaded with shared_preload_libraries.  This is necessary to
be able to update the test introduced in 768a9fd5535f to use the macros
INJECTION_POINT_{LOAD,CACHED}() rather than a SQL function in the module
injection_points forcing a load, as this test runs a callback in a
critical section where no memory allocation should happen.

Initializing the shared memory state of the module while loading
provides a strict control on the timing of its allocation.  If the
module is not loaded at startup, it will use a GetNamedDSMSegment()
instead to initialize its shmem state on-the-fly.

Per discussion with Álvaro Herrera.

Author: Michael Paquier
Discussion: https://postgr.es/m/ZsUnJUlSOBNAzwW1@paquier.xyz