optimize left edge finding in autotune

use rlock for error notification

use rwlock for sessions object

Update dependencies (xtaci#182)

* Git ignore go vendor folder

* Update go dependencies

flush when input has make a window slide

returns current RTO

Import /v5 in example code. (xtaci#170)

This enables the removal of a `kcp-go v5.4.20+incompatible` from go.mod.
(Automatically using `go mod tidy`.)

Co-authored-by: David Fifield <[email protected]>

properly drain timer.C

optimize on timedsched

bugfix in timer