Remove global variables, use proper class/OOP encapsulation

[vietlq]

The file and some files have quite a few global variables:

https://github.com/melonproject/oyente/blob/3fc108ca550aa5270ea5368fdb685b94f0e17450/oyente/symExec.py

Global variables introduce unnecessary complexity and signal poor design/security. This can be solved by proper OOP encapsulation and treat them as class data members. This also allows you to run multiple verifiers in parallel within the same Python process.

[bonedaddy]

IMO this should be completely rewritten from python to golang, or include mult-threaded processing.