Escape colons in BagOStuff key segments

Escape colons in BagOStuff key segments

For the sake of safety and correctness, the following BagOStuff::makeKey()
invocations should return distinct keys:

   $cache->makeKey( 'ab:', 'cd' );
   $cache->makeKey( 'ab', ':cd' );

That is not currently the case, because while we use ':' as a key path
separator, we don't escape ':' in the input supplied to makeKey(). So, make
BagOStuff::makeKeyInternal() URL-encode colons.

To prevent this from messing up the max. key length calculations, reproduce
this logic in MemcachedBagOStuff::makeKeyInternal(), in lieu of having the
method call its parent.

Change-Id: I83ea7e7336a1c9e64aa42284c2517089a736efe5