- only for equality matching
- only components of ASN.1 types used in an X.509 certificate can be indexed
(composite types except for RDN and DN are not supported yet)
- how to use
index [attribute name].[component reference] eq
Ex)
index userCertificate eq
index userCertificate.toBeSigned.issuer.rdnSequence eq
index userCertificate.toBeSigned.serialNumber eq
index userCertificate.toBeSigned.version eq

Add better approx test

  always use DB_AUTO_COMMIT when opening a database, don't make it
  dependent on an in-progress transaction

  use mutex instead of DB lock for db access.

by other values of the attribute.

Also, filter out duplicate keys to avoid redundant DB operations.

Key collisions due to separate attributes indexed by supertype are still
not safe. Yuck.

of #ifdef USE_SL_MALLOC...