Dynamically change thread sleeping time to increase throughput during high load

Previously we were always blocking for 50ms no matter what, but now
we'll be dynamically adjust our block ratio depending on the current
clipboard load. In the worst case scenario when user used the clipboard only
once will go with the next sequence: 0 1(16 times) 1 2 4 8 16 32 50 50
50 and so on, until the next user request.

CHANGELOG.md

@@ -3,6 +3,7 @@
 ## Unreleased
 
 - Perform loaded data normalization for text/plain;charset=utf-8 mime type
+- Fix clipboard throttling
 
 ## 0.3.5 -- 2019-09-3