message: Add NEED_CONTEXT_ALL message type

GstElement can store multiple persistent context even if they are
all identical type. But GstBin will response the very first context
from NEED_CONTEXT message. This NEED_CONTEXT_ALL message will be used
for requesting all available specific contexts.
28 jobs for need-context-all in 51 minutes and 22 seconds (queued for 2 seconds)
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