SCHEMA-411. Commands not being logged because checkin server URL was null.

Since the BENTO_CHECKIN_SERVER wasn't being exported, the checkin client was
getting a null reference to the environment variable hence the command wasn't
being logged.
SCHEMA-409. Add caching of AvroCellEncoders to table writers.

issue: https://jira.kiji.org/browse/SCHEMA-409
review: https://review.kiji.org/r/623
SCHEMA-401. Update cell write path validation

issue: https://jira.kiji.org/browse/SCHEMA-401