[LIVY-998][THRIF] Support connecting to an existing sessions using session name

[askhatri]

Support connecting to an existing session using the session name via Thrift Server.

What changes were proposed in this pull request?

JIRA: https://issues.apache.org/jira/browse/LIVY-998

The Thrift Server can set the session name, but it cannot use an existing session name and connect to it. Added the fix to use an existing session name and connect to it via Thrift Server.

How was this patch tested?

Created a session using session name:

curl --no-progress-meter -X POST -H "Content-Type: application/json" --data '{"kind": "spark", "name": "sesssionName"}' http://HOST:PORT/sessions

Connected to the existing session via Thrift Server using:

$ beeline -u "jdbc:hive2://HOST:PORT/?livy.session.name=sesssionName"

[gyogal]

LGTM, thanks, Asif! A minor nitpick I have is there is a typo in the commit message, but that can updated fixed when the change is merged.

[askhatri]

Thank you @gyogal , I have updated the commit message now.