Add DROP_CLIENT env variable (#432)

This will disconnect any client based on their version string,
using a regular expression.
Useful for dropping buggy/unsupported clients.

environment.rst

@@ -201,6 +201,12 @@ These environment variables are optional:
 
   If you are not sure what this means leave it unset.
 
+.. envvar:: DROP_CLIENT
+
+  Set a regular expression to disconnect any client based on their
+  version string. For example to drop versions from 1.0 to 1.2 use
+  the regex ``1\.[0-2]\.\d+``.
+
 
 Resource Usage Limits
 =====================