ACCUMULO-2663 - Remove usage of String#split from Combiner#init

Noticed that inside Combiner#init, the comma-separated list of columns is split out using the inefficient String#split method. This should be switched into something that doesn't used Regexs since this method is invoked a surprising amount in normal usage.

Signed-off-by: Josh Elser <elserj@apache.org>
Merge branch '1.5.2-SNAPSHOT' into 1.6.1-SNAPSHOT