fix short opts -s (#285)

* fix: collision short option '-s'

Short opt '-s' is used for both `--shortids` and `--size`

Now it is for `--size` only

* doc: update

* doc: update README for usages.

* chore: remove duplicate `conn_rate`

* chore: remove dup '--connrate'

Author: qzhuyan

CHANGELOG.md

@@ -3,6 +3,8 @@
 ## 0.4.34
 
 * new tls1.3 opt for Key exchange alg: `-keyex-algs` 
+* short opt `-s` is now for `--size` only, it was shared by `--shortids`
+* update usages in README.md
 
 ## 0.4.33