r1149: implemented random open syncmer

On the mm2-update dataset, -j8 leads to sparser k-mer selection at higher
accuracy. The speed becomes a little slower. There seems a benefit but not a
big one.

minimap2.1

@@ -79,6 +79,19 @@ Minimizer k-mer length [15]
 .BI -w \ INT
 Minimizer window size [10]. A minimizer is the smallest k-mer
 in a window of w consecutive k-mers.
+.TP
+.BI -j \ INT
+Syncmer submer size [10]. Option
+.B -j
+and
+.B -w
+will override each: if
+.B -w
+is applied after
+.BR -j ,
+.B -j
+will have no effect, and vice versa.
+
 .TP
 .B -H
 Use homopolymer-compressed (HPC) minimizers. An HPC sequence is constructed by