[sonic-swss-common] Update submodule (#8246)

Includes below commits

a90529f 2021-07-22 | Fix DBInterface blocking operations (#505) [Qi Luo]
e6a0a56 2021-07-20 | Reimplement swig properties (#504) [Qi Luo]
a0ed7a0 2021-07-19 | modify the logger for more flexable change of log level (#498) [emil mihaylov]