Disable ns caching by default

See
clojure-emacs/cider@122a40f
for details.

TLDR - people would change their ns from time to time and forget to invalidate
the cached version. As I can't come up with a smart invalidation strategy I
decided to limit the scope of the caching instead.

Author: bbatsov

CHANGELOG.md

@@ -5,6 +5,7 @@
 ### Changes
 
 * Require Emacs 25.1+.
+* Disable ns caching by default.
 
 ## 5.8.1 (2018-07-03)
 

clojure-mode.el

@@ -1819,7 +1819,7 @@ content) are considered part of the preceding sexp."
       (zero-or-one (any ":'")) ;; (in-ns 'foo) or (ns+ :user)
       (group (one-or-more (not (any "()\"" whitespace))) symbol-end)))
 
-(defcustom clojure-cache-ns t
+(defcustom clojure-cache-ns nil
   "Whether to cache the results of `clojure-find-ns'.
 
 Note that this won't work well in buffers with multiple namespace