[hw_escpos]  Serialize access to the printer

This change serializes access to the printer, avoiding lots of weird errors because of concurrent or out of order accesses to it...

Since I don't have clear understanding on what you are doing with the original "lock" present, I added a "lock2" variable, and did serializing with it.

If you could explain what is the intend of lockedstart, I could possibly avoid this new variable and just use the original one...

Let me know and I will gladly change it...

Regards, 
-Mario