Allow serial number to be a string

[vzahradnik]

Is your feature request related to a problem? Please describe.
I am considering using UUID for device identification and I would like to set the UUID as the serial number in the TagVal header. Currently it is expected the serial number to be a long int. I propose to extend this field to support arbitrary strings.

Describe the solution you'd like
Encode long integers into strings and then parse the strings at the remote end. If parsing fails, assume string was used.

Additional context
This change has to be implemented in all our SDKs. Please keep this issue open for tracking purposes.