Make the library reentrant and thread-safe

[Mek101]

Moves non-immutable variables from class-scope to function scope, and makes the constants shared across all class instances.
This allows to make multiple concurrent calls to OpenGraphParser.parse without having different threads and coroutines interfere with each other

[pknotfound]

@Mek101
These changes were locally committed and were being tested out currently.
Your commit, however, includes moving constants to be shared across instances.

I shall include your changes.

Thank you for your contribution.