Reduce posix.Serial.read1 calls

[tiagocoutinho]

The posix implementation of _read() calls _read1() for each byte received.
The issue is that read1 is quite expensive because it has create an event object and register it as a reader in the event loop.
(See https://github.com/tiagocoutinho/serialio/blob/master/serialio/aio/posix.py#L36)

A better option would be to launch a task in the background reading from the fd and storing read requests in the buffer.
This task would be launched in the Serial constructor so there would be one task per serial connection.

FYI, this is what is done for RFC2217 and also indirectly in the raw tcp connection.

It is not a bug but a good enhancement