Virtual tables are kept in a thread_local registry for deduplication
purposes. The problem is that thread_local variables are destroyed late,
possibly after the schema registry and the reactor are destroyed.
Currently this isn't a problem, but after a seastar change to
destroy the reactor after termination [1], things break.
Fix by moving the registry to system_keyspace. system_keyspace was chosen
since it was the birthplace of virtual tables.
Pimpl is used to avoid increasing dependencies.
[1] 101b245ed7
This is a readability refactoring commit without observable changes
in behaviour.
initialize_virtual_tables logically belongs to virtual_tables module,
and it allows to make other functions in virtual_tables.cc
(register_virtual_tables, install_virtual_readers)
local to the module, which simplifies the matters a bit.
all_virtual_tables() is not needed anymore, all the references to
registered virtual tables are now local to virtual_tables module
and can just use virtual_tables variable directly.
The definitions of virtual tables make up approximately a quarter of the
huge system_keyspace.cc file (almost 4K lines), pulling in a lot of
headers only used by them.
Move them to a separate source file to make system_keyspace.cc easier
for humans and compilers to digest.
This patch also moves the `register_virtual_tables()`,
`install_virtual_readers()` as well as the `virtual_tables` global.
Closes#14308
