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Some portions of the test suite did not correctly handle asyncmeta, fix
those issues.  Tests execute and pass after this change

This commits deletes all references and code for back-bdb and back-hdb.
There is some follow up work still necessary to flush out the admin
guide for back-mdb.

Nadezhda Ivanova authored Feb 18, 2019 and Quanah Gibson-Mount committed Feb 28, 2019

Includes all the changes necessary to fix back-asyncmeta issues
discovered during on-site testing since the start of 2016.
These include:
Issues with stability - crashes and assetion failures
Incorrect behavior during unstable network conditions, such as inability to reset connections
or process responses, or "hanging" to wait for a response that would never be received.
Memory leaks and memory management fixes - major redesign of the way back-asyncmeta
works with memory contexts.
Rewrite was replaced with suffix-massage in configuration, and the network-timeout value was changed to milliseconds.
Incorrect behavior when SASL is used to bind to a target.
Many problems caused by race conditions
Fixes for compiler warnings, and tests.
Cleanup of unused code.

Does not update the tests themselves.

In configure --help, call it "mdb database backend" instead of something
with "memory-mapped" which could confuse somebody about a memory-only DB.

Conflicts:

	servers/slapd/back-mdb/Makefile.in

Simplifies tests for the DB storage backends.
Adds indexing etc to ndb in some cases, to match bdb/hdb.
This also fixes some broken back-null/back-ldif settings.

allow each (back-config) syncrepl test to use the preferred sync type, but allow changing it via the -s switch for ./run