Problem: sending any Whisper message fails

The error is "PoW too low to compete with other messages"

This has been previously reported in #7144

Solution: prevent the move semantics

The source of the error is in PoolHandle.relay
implementation for NetPoolHandle.

Because of the move semantics, `res` variable is in fact
copied (as it implements Copy) into the closure and for
that reason, the returned result is always `false.