Eina Debug: fix a bug resulting in registering opcodes twice

Eina Debug: fix a bug resulting in registering opcodes twice

The opcodes registration request is sent directly in case the connection
is already made. Otherwise, the request is waiting for the connection to
be made by the dedicated thread (not the main loop).
That's why the request can be sent by the two different threads at the
same time, leading to send it twice. It means a callback for an opcode
would be invoked twice everytime a request with this opcode is received.

This patch fixes it by checking if the request has already been sent.