Ensure db/table names are always case insensitive in catalog topic entry keys

This fixes a bug that can happen with 'invalidate metadata <table name>' if the following
sequences of events happens:

1) Table is created in Impala (table names are always treated as lower case)
2) Table is dropped and re-created in Hive, using the same name but different casing
3) invalidate metadata <table name> is run in Impala, which will update the existing
   table with the version from the Hive metastore.

When building the next statestore update, the catalog server will send an update out
thinking that the table from 1) was dropped and the table from 3) was added because
the topic entry key is case sensitive. This may incorrectly remove the table from
an impalad's catalog. The fix is to always treat db/table names as case insensitive.

Change-Id: Ib59edc403989781bf12e0405c0ccd37b8e41ee41
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1634
Tested-by: jenkins
Reviewed-by: Lenni Kuff <lskuff@cloudera.com>