Good news is: The latest SPS on PI 7.1, 7.11 and 7.3 have introduced a mechanism to send the EOIO messages of a queue to a single J2EE engine and reducing the need for cluster communication. Bad news is: Due to a bug, this mechanism is not working well now and SAP is working on fixing it.
It won't take time for SAP to come up with a better mechanism, meanwhile you can disable this (default) feature (on the above said SPS) by setting AE_EOIO_OPTIMIZE to 0 in SXMB_ADM --> Integration Engine Configruation. To enable it, you can set the same parameter to 1.
The error (503 service not available. Detail: server overload: no more sessions available) details are available on SAP Note 1586827.
Recommended reading for SAP XI/PI developers:
SAP NetWeaver PI Development: Practical Guide
With this practical guide, you ll learn how to develop mappings, adapters, and proxies for SAP NetWeaver Process Integration. Each exercise includes detailed descriptions for all development and configuration objects (including monitoring), and then concludes with an integration example. The book is suitable for SAP NetWeaver PI beginners.