output/srzip: queue samples before ZIP operation

Accumulate samples from multiple session feed packets before sending
them off to ZIP archive operations. This improves throughput for those
setups where acquisition devices or input modules provide only few
samples per session feed send call.

This version also splits large packets from applications into smaller
ZIP members (if the application's packet size is larger than the output
module's local buffer size). If that is not desired, the implementation
needs adjustment to immediately pass larger blocks to ZIP operations
(after potentially flushing previously queued data) instead of looping.

This fixes bug #974.