dnsdist: Prevent a race when calling `registerWebHandler` at runtime

dnsdist: Prevent a race when calling `registerWebHandler` at runtime

The `registerWebHandler()` Lua method can be used to tie a custom Lua
function to an HTTP endpoint. This function was clearly not intended
to be used at runtime but this was never enforced, so let's prevent
a race condition by wrapping the internal web handlers map in a lock.