allow scanning based on keys attributes

[romange]

Add another filter to the SCAN command:

ATTR <mask>

to be able to scan keys based on additional attributes:
mask may contain the following attributes:

1. v - volatile, keys that have ttl
2. p - permanent, keys that do not have ttl
3. a - accessed, the key has been accessed since the last load/flush event, or the last time a flag was reset.
4. u - untouched, the key has not been accessed/touched.

in the future:

• d the value resides on disk
• m the value resides in memory

for (3), (4) we need to introduce an additional flag in CompactObject to track it on keys (note that usually the flags keys are set on values).
Also we will need a DEBUG ACCESS RESET to reset a attributes for the current database.

Note that we already provide undocumented "BUCKET " filter for SCAN.