stream: introduce min inspect depth logic

stream: introduce min inspect depth logic

Some rules need to inspect both raw stream data and higher level
buffers together. When this higher level buffer is a streaming
buffer itself, the risk of mismatch exists.

This patch allows an app-layer parser to set a 'min inspect depth'.
The value is used by the stream engine to keep at least this
depth worth of data, so that the detection engine can request
all of it for inspection.

For rules that have the SIG_FLAG_FLUSH flag set, data is inspected
not from offset raw_progress, but from raw_progress minus
min_inspect_depth.

At this time this is only used for sigs that have their fast_pattern
in a HTTP body and have raw stream match as well.