Avoid infinite loop in DeltaManager with invalid batch size

[lihongyi87]

Problem

When sendAllSessions is false and sendAllSessionsSize is configured to zero or a negative value, the batch loop in handleGET_ALL_SESSIONS never terminates:

for (int i = 0; i < currentSessions.length; i += getSendAllSessionsSize()) {

• With sendAllSessionsSize == 0: i never advances, so the loop runs indefinitely, sending empty session arrays and sleeping on each iteration. This blocks the cluster receiver thread on the master node and prevents EVT_ALL_SESSION_TRANSFERCOMPLETE from ever being sent.
• With sendAllSessionsSize < 0: i decreases, and new Session[len] throws NegativeArraySizeException.

The setSendAllSessionsSize setter performs no validation, and the documentation (cluster-manager.xml) does not state a minimum value.

Fix

When sendAllSessionsSize is not positive, fall back to sending all sessions in a single batch (the same behaviour as sendAllSessions == true):

if (isSendAllSessions() || getSendAllSessionsSize() <= 0) {

This is a one-line change at the point of use, keeping the setter unmodified for consistency with the rest of DeltaManager.

[markt-asf]

I'd rather document that sendAllSessionsSize must be a positive integer and throw IllegalArgumentException if it isn't.

[lihongyi87]

Thanks for the review. Reworked as suggested: the setter now rejects a non-positive sendAllSessionsSize with an IllegalArgumentException, the documentation states the value must be a positive integer, and the fallback in handleGET_ALL_SESSIONS has been reverted. Tests updated accordingly.

[markt-asf]

This needs to be below the line since it will be back-ported.

[lihongyi87]

Moved below the line. Thanks for the pointer.