dnsdist: Prevent the cache ptr from being altered under our feet

dnsdist: Prevent the cache ptr from being altered under our feet

Make sure we hold the Lua mutex before getting the packet cache
shared_ptr, so that we don't have a thread reading it at the
exact same time it is altered by another.
We could have used atomic_load/atomic_store but libstdc++ uses
a pool of mutex for that anyway.
This might fix #3396.