gdbus: use larger buffer for larger reads from socket

When reading a new message from a socket used for D-Bus, the first read was 16 bytes to include message size, and then the exact message size was read into buffer.

Now, data is read into a buffer of default size 4096 bytes. It gives some chance that the data already includes whole messages, and some of subsequent polls and reads are not needed anymore.

This speeds up receiving of messages.