Implement Operation Patient $purge

[alexanderkiel]

• we like to purge all resources in the compartment of a particular Patient
  • however the Patient compartment contains some references like Group.member that should not mean to delete the Group
  • so for now we will only use patient and subject references because those resources linked by both should be purgeable without problem
• we need a new transaction operator called purge with the same parameters like delete which are type and id
  • add specs in blaze.db.spec
• we need also a new transaction command called purge without if-match (optimistic locking) bot otherwise identical to delete
  • add specs in blaze.db.tx-log.spec
• inside transaction handling done in blaze.db.node.tx-indexer/index-tx we will expand the purge command into individual delete and delete-history commands

This issue is part of the Epic: #399