The header sits in many other headers, but there's a handy
schema_fwd.hh that's tiny and contains needed declarations
for other headers. So replace shema.hh with schema_fwd.hh
in most of the headers (and remove completely from some).
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>
Message-Id: <20200303102050.18462-1-xemul@scylladb.com>
Requires "workaround" fix for schema_registry and frozen_mutation, since
the former is a free-float thread local, and the latter is a pure data
carrier. frozen_schema can take a parameter for unfreeze, but schema
registry requires being told which the system extensions are.
