[YCQL] Correctly set release_version for system.peers queries

[m-iancu]

Currently the release_version column exists in the system.peers table and has the right type, but the value is not being set when producing the result for queries. Therefore its value is always returned as null.

Example

Start a cluster locally with:

./bin/yb-ctl create --rf 3

Connect with ycqlsh

./bin/ycqlsh

Check release version in system.local (should be set correctly)

select key, broadcast_address, release_version from system.local ;

 key   | broadcast_address | release_version
-------+-------------------+-----------------
 local |         127.0.0.1 |    3.9-SNAPSHOT

Check release version in system.peers (will be null)

select peer, release_version from system.peers;

 peer      | release_version
-----------+-----------------
 127.0.0.3 |            null
 127.0.0.2 |            null

Relevant code

The YCQL system tables are implemented as virtual tables, and the results for them are constructed in code.
Specifically for system.local the release version is set in yql_local_vtable.cc:LocalVTable::RetrieveData.
On the other hand for system.peers the column is indeed created in yql_peers_vtable.cc:PeersVTable::CreateSchema.
But its value is not set in yql_peers_vtable.cc:PeersVTable::RetrieveData.
Fix should be to set the value accordingly (also the value "3.9-SNAPSHOT" should probably become a const in a shared place like yql_local_vtable for code cleanliness).

Testing

The main test for YCQL system tables is TestSystemTables.java so we should add a test for this there (probably extend an existing test to check this value).
Note: the test can be ran using:

ybd --java-test org.yb.cql.TestSystemTables

[tedyu]

release-ver.txt

Came up with a patch.

Trying to compose a test next.