cmake,rgw: use jaeger-base target, as a dependency for building dbstore

cmake,rgw: use jaeger-base target, as a dependency for building dbstore

* dbstore also has tracing dependencies and missing linking for libraries
* use `using` keyword in common/tracer.cc to silence unused method
  warnings
* log building of external project jaegertracing, opentracing into build
  file, (done to reduce external project build verbosity)

Signed-off-by: Deepika Upadhyay <dupadhya@redhat.com>