For use with back-ldap/back-meta/syncrepl/etc

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.

Follow on to commit 88d22a1c

Remove dead code

We're using PKCS#8 syntax, drop the OpenLDAP syntax OID.
Rename attribute accordingly.
Tweak validator to accept encrypted keys.

Josh Soref authored Feb 26, 2017 and Quanah Gibson-Mount committed Oct 11, 2017

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* beg...

SLAP_AUXPROP_DONTUSECOPY is ok for release

Tell frontend the op will finish asynchronously, leave it alone

Show time spent in conn+threadpool queues before an op actually executes.
Also clean up timestamp handling

So any initializers in code predating sc_writewait
will find sc_private in the expected position.

Store attrs with a large number of values separately from the
main entry blob. Note - we need support for large DUPSORT values
for this to be generally usable.

Don't need op->o_hr_time, just use o_tincr, that's what it
was intended for anyway. Use "etime=" like other products do.
Simplify ifdefs. Use gettimeofday, it's always available now.

Only in back-mdb; back-bdb needs work but it's deprecated;
not worth the effort. In particular txn, retry after deadlocks
makes the whole thing too messy.

Invoked before a blocked writer waits for socket to be writable.
Use in back-mdb to release reader txn while waiting.

Currently only implemented for OpenSSL.
Needs an option to set the criticality flag.

Do not use. backend/overlay code needs to be fixed accordingly.
In particular, code which currently frees configuration in _db_close()
must be changed to free only in _db_destroy().

After restructuring, it must be possible to call _db_close()/_db_open()
multiple times in succession without requiring _db_config() in between.