[add] managers/certificate.py with CertificateManger class
[mv] creating self-signed certificate logic into CertificateManger class
[add] renew and days_before_expire methods to CertificateManger class
[mv] several certificate dependent constants to managers/certificate.py
[add] CherryPy BackgroundTask to invoke certificate check hourly (3600 secs)
[fix] tests
[fix] bug with txn timer clean (clean_txn_by_timeout, worker and invoking of it)
