When active defrag kicks in and finds a big list, it will create a bookmark to
a node so that it is able to resume iteration from that node later.

The quicklist manages that bookmark, and updates it in case that node is deleted.

This will increase memory usage only on lists of over 1000 (see
active-defrag-max-scan-fields) quicklist nodes (1000 ziplists, not 1000 items)
by 16 bytes.

In 32 bit build, this change reduces the maximum effective config of
list-compress-depth and list-max-ziplist-size (from 32767 to 8191)

... with CSC disabled

This fixes WATCH and client side caching with keys expiring because of
a synchronous access and not because of background expiring.

This helps in avoiding multiple definition of this variable, its also
defined globally in sds.c
Signed-off-by: NKhem Raj <raj.khem@gmail.com>

Signed-off-by: Nlifubang <lifubang@acmcoder.com>

1. Call emptyDb even in case of diskless-load: We want modules
   to get the same FLUSHDB event as disk-based replication.
2. Do not fire any module events when flushing the backups array.
3. Delete redundant call to signalFlushedDb (Called from emptyDb).

Signed-off-by: Nlifubang <lifubang@acmcoder.com>

Because "keymiss" is "special" compared to the rest of
the notifications (Trying not to break existing apps
using the 'A' format for notifications)

Also updated redis.conf and module.c docs

SELECT, and HELLO are commands that may be executed by the client
as soon as it connects, there's no reason to block them, preventing the
client from doing the rest of his sequence (which might just be INFO or
CONFIG, etc).

MONITOR, DEBUG, SLOWLOG, TIME, LASTSAVE are all non-data accessing
commands, which there's no reason to block.

the warning condition was if usage > limit (saying it'll cause eviction
or oom), but in fact the eviction and oom depends on used minus slave
buffers.

other than fixing the condition, i add info about the current usage and
limit, which may be useful when looking at the log.

currently there's no bug since the flags these functions handle are
always lower than 32bit, but still better fix the type to prevent future
bugs.

using panic rather than exit means you get s stack trace of the code
path that experianced the error, and possibly other info.

this function possibly iterates on the module list