When it exists configured matching rule for an indexed attribute it should be used if the schema does not define one

[tbordaz]

Issue Description
An attribute is defined in the schema. The matching rule is optional in the schema so the definition falls back to the default matching rule that is lexicographic order.

An attribute can be configured to be indexed and the configuration can define a matching rule 'nsMatchingRule".

If the schema does not define a matching rule, the server should fall back to the configured one ('nsMatchingRule') prior to default lexicographic one

Package Version and Platform:
All versions

Steps to Reproduce
Will be provided in a testcase

Expected results
Configured MR should be the default if the schema does not define one