Implementation of sample streaming mode

[macikgozwa]

Implementation of sample streaming mode per https://github.com/openconfig/reference/blob/master/rpc/gnmi/gnmi-specification.md#35152-stream-subscriptions

What is NOT included:

• TARGET_DEFINED mode.
• suppress_redundant and hence heartbeat_interval are ignored.
• The behavior of dial-out streaming hasn't changed. It will stay ON_CHANGE.

For testing results, see the example queries and the response:

• Virtual DB Single table:

./gnmi_cli_py/py_gnmicli.py -g -t localhost -p 8080 -m subscribe --submode 2 -x "COUNTERS/Ethernet0"   -xt COUNTERS_DB -o "ndastreamingservertest" --interval 5000 --update_count 3

Response: single-table.txt

• Virtual DB Multiple table:

./gnmi_cli_py/py_gnmicli.py -g -t localhost -p 8080 -m subscribe --submode 2 -x "COUNTERS/Ethernet*"   -xt COUNTERS_DB -o "ndastreamingservertest" --interval 5000 --update_count 3

Response: multi-table.txt

• Single field

./gnmi_cli_py/py_gnmicli.py -g -t localhost -p 8080 -m subscribe --submode 2 -x "COUNTERS/Ethernet0/SAI_PORT_STAT_ETHER_IN_PKTS_128_TO_255_OCTETS"   -xt COUNTERS_DB -o "ndastreamingservertest" --interval 5000 --update_count 3

Response: single-field.txt

• Single field across multiple tables

./gnmi_cli_py/py_gnmicli.py -g -t localhost -p 8080 -m subscribe --submode 2 -x "COUNTERS/Ethernet*/SAI_PORT_STAT_ETHER_IN_PKTS_128_TO_255_OCTETS"   -xt COUNTERS_DB -o "ndastreamingservertest" --interval 5000 --update_count 3 

Response: multi-field.txt

• Virtual DB multiple table with update_only where only the changed part is included in the update: updates-only.txt