void returnSession(SessionType session) {
    returnSessionInternal(session, false);
  }
The number of sessions that needs to be started or killed because of the resize calls on
the pool. When increasing the size, we set this to a positive number and start new sessions
on background threads, gradually bringing it back to 0.
When decreasing the size, we try to kill as many existing sessions as we can; if that is
not enough because the sessions are in use or being restarted, we kill them as they are
re-added to the pool.
Repeated calls to resize adjust the delta to ensure correctness between different resizes.
ql/src/java/org/apache/hadoop/hive/ql/exec/tez/TezSessionPool.java