Make all backend operations transaction aware
    
    All backend functions will use the parent transaction (if any) to perform
    all database read/write operations, and for the creation of nested
    transactions.  The pseudocode looks like this:
    begin operation
    grab parent txn from pblock SLAPI_TXN and save to a local variable
    use this parent txn to perform any searches or other ops before the
      nested txn is created (if created - search ops do not do this)
    create the nested txn (if a write op) using the parent_txn
    stash the new txn in the pblock SLAPI_TXN
    call the betxnpreop plugins
    perform the backend ops inside the txn
    call the betxnpostop plugins
    commit the nested txn
    restore the saved parent_txn to the pblock SLAPI_TXN
    upon failure, abort the nested txn and restore the saved parent_txn
      to the pblock SLAPI_TXN
    
    There were a few api changes to allow the txn to be passed in wherever
    it is needed.
    The database lock was changed from a mutex to a monitor to allow plugins
    to call other plugins inside the same parent transaction.
    Reviewed by: nhosoi (Thanks!)