Creates a new session. A session can be used to perform transactions that read and/or modify data in a Cloud Spanner database. Sessions are meant to be reused for many consecutive transactions. Sessions can only execute one transaction at a time. To execute multiple concurrent read-write/write-only transactions, create multiple sessions. Note that standalone reads and queries use a transaction internally, and count toward the one transaction limit. Active sessions use additional server resources, so it's a good idea to delete idle and unneeded sessions. Aside from explicit deletes, Cloud Spanner can delete sessions when no operations are sent for more than an hour. If a session is deleted, requests to it return NOT_FOUND. Idle sessions can be kept alive by sending a trivial SQL query periodically, for example, "SELECT 1".