[submodule] Advance sonic-swss-common pointer

Update sonic-swss-common submodule pointer to include the following:
* 6b6842a [NotificationProducer] add pipeline support ([sonic-net#708](sonic-net/sonic-swss-common#708))
* 2cb5ea0 Increase the netlink buffer size from 3MB to 16MB. ([sonic-net#739](sonic-net/sonic-swss-common#739))
* dacbdad RedisPipeline ignore flush when call dtor from another thread. ([sonic-net#736](sonic-net/sonic-swss-common#736))

Signed-off-by: dprital <drorp@nvidia.com>