Update sonic-swss-common submodule (#8565)

508202b 2021-08-24 Add a new field for FLEX_COUNTER_TABLE to indicate delay for flex counters (#523)
9fd7dbf 2021-08-20 [logger] Make map access thread safe and proper terminate thread (#510)
e4c3d0b 2021-08-20 Test with coverage output, publish gcov on Azure pipelines (#517)
ef21bec 2021-08-18 Update schema.h to include config_db DHCP and state_db counter table (#521)
4e4eb9d 2021-08-19 [schema] Add next hop group table to schema (#475)

Signed-off-by: Shlomi Bitton <[email protected]>