Fix issue w/ TCP reassembler not delivering some segments.

For example, if we have a connection between TCP "A" and TCP "B" and "A"
sends segments "1" and "2", but we don't see the first and then the next
acknowledgement from "B" is for everything up to, and including, "2",
the gap would be reported to include both segments instead of just the
first and then delivering the second.  Put generally: any segments that
weren't yet delivered because they're waiting for an earlier gap to be
filled would be dropped when an ACK comes in that includes the gap as
well as those pending segments.  (If a distinct ACK was seen for just
the gap, that situation would have worked).

Addresses BIT-1246.

testing/btest/scripts/base/protocols/http/entity-gap2.bro

@@ -0,0 +1,24 @@
+# @TEST-EXEC: bro -r $TRACES/http/entity_gap2.trace %INPUT
+# @TEST-EXEC: btest-diff entity_data
+# @TEST-EXEC: btest-diff extract_files/file0
+
+global f = open("entity_data");
+global fn = 0;
+
+event http_entity_data(c: connection, is_orig: bool, length: count,
+                       data: string)
+	{
+	print f, data;
+	}
+
+event content_gap(c: connection, is_orig: bool, seq: count, length: count)
+	{
+	print f, fmt("<%d byte gap>", length);
+	}
+
+event file_new(f: fa_file)
+	{
+	Files::add_analyzer(f, Files::ANALYZER_EXTRACT,
+					   [$extract_filename=fmt("file%d", fn)]);
+	++fn;
+	}