Fix MIPS cache coherency on Linux

MIPS chips require manual control of on-chip caches. The cacheflush
syscall being used here only exists on MIPS Linux, other OSs will
require revisiting.
ITS#7908
Add olcSssVlvMaxPerConn to the olcSssVlvConfig objectClass
ITS#7908
ITS#7909 fix slap_callback init
ITS#7909
Remove 7909, not released
ITS#7913 mdb_cursor_get(*_DUP) cleanup

FIRST_DUP/LAST_DUP should succeed even on non-duplicate values
PREV_DUP should reset cursor's EOF flag.
Merge remote-tracking branch 'origin/mdb.master' into OPENLDAP_REL_ENG_2_4
ITS#7912 fix index config
ITS#7912
More for MIPS

Paranoia for 3630066843b7ca6b2cd12911d3e2fe3314cd4549 do the
cacheflush before setting mti_txnid.
ITS#7793 update branch key if needed
ITS#7793 doc update, again
ITS#7793 more for branch key update
More for MIPS

Do a cacheflush after writing data pages, separate from meta's cacheflush
ITS#7895 drop toolmode checks in str2entry

They were obsoleted by ITS#6737 and have been redundant ever since.
(commit 15f1e7bdb7ae487424f19934b0d5dbf4bc30bea6)
ITS#7915 fix mdb_entry_release

In server mode, make sure the entry was actually ours
ITS#7915 fix translucent_modify

Go directly to local backend when fetching local entry
ITS#7895, ITS#7915
ITS#7906 better fix

don't free pwcons unless we're actually destroying this DB