CRM/Logging - Fix various bugs in schema parsing

CRM/Logging - Fix various bugs in schema parsing

This fixes a couple of bugs in the schema parsing methods used by
Civi's extended logging feature:

- CRM_Logging_Schema::getIndexesForTable only queried for constraints,
  not returning any indexes.
- CRM_Logging_Schema::getIndexesForTable returned an array in the form
  [0 => ['constraint_name' => 'foo']] rather than the expected array
  of index names (i.e. ['foo']).
- CRM_Logging_Schema::columnSpecsOf contained an off-by-one error and
  a wrongly used substr parameter causing column lengths to include
  surrounding parenthesis. This would result in a "varchar(42)"
  column returning a length of "(42)" instead of "42".