Currently, because serialize_visitor::operator() is not implemented
for counters, we cannot convert a counter returned by a WASM UDF
to bytes when returning from wasm::run_script().
We could disallow using counters as WASM UDF return types, but an
easier solution which we're already using in Lua UDFs is treating
the returned counters as 64-bit integers when deserializing. This
patch implements the latter approach and adds a test for it.
b25ee62f75