`cider-jump-to-var` is broken when there is no symbol at point

Symbol is taken from `(cider-read-symbol-name "Symbol: " 'identity)` which doesn't return anything because it's asynchronous.  

Also, make `cider-jump-to-var` and `cider-stacktrace-navigate` use common workhorse as they were using almost identical code. 

This PR is based on #760 as it touches the same code.