Support for binary data for DBConnector::hgetall

[gpunathilell]

Currently we cannot read binary data using the get_all function in python for a SonicV2Connector. Similar to the fix done for Table::get in #836 a fix is required for DBConnector::hgetall function which would allow support for binary data for the SonicV2Connector in Python.
To support binary data that can contain null bytes, the redis reply str is passed along with its len into the std::string(char*,size_t) constructor that supports the input size.