DB Access: DB scan cursor support for keys and fields to iterate it

[mbalachandar]

Added following methods to scan and iterate the db entries using redis DB scan cursor.

• NewScanCursor() : to create ScanCursor type to perform the GetNext.. operation
• GetNextKeys() : retrieves a few keys and returns true if the scan is complete.
• GetNextFields() : retrieves a few matching fields and returns true if the scan is complete.
• GetNextRedisKeys() : retrieves a few redisKeys and returns true if the scan is complete

Required for the subscription enhancements as described in HLD sonic-net/SONiC#1287

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: a-barboza (80190b9, b1430d0)
• ✅ login: mbalachandar / name: Balachandar Mani (dea01d0, 688d7f7, 6c5ff49)

[tomek-US]

LGTM (based on Ryan's review)