add support for DeleteAllKeys update operation

[cmumford]

Original issue 43 created by josephwnorton on 2011-10-01T05:55:50.000Z:

It would be handy to have a new update operation that can delete all keys atomically without having to close, to delete, and then to re-open the database.

[cmumford]

Comment #1 originally posted by sawloo on 2012-09-04T22:21:48.000Z:

something like Delete(start, limit) or Delete(range) will be great for me

[cmumford]

Comment #2 originally posted by sanjay@google.com on 2012-09-18T16:43:09.000Z:

I think we will not add this operation to leveldb since a helper routine
that accomplishes the same thing with equivalent performance can be
built outside leveldb.

[genki]

@cmumford You mean deleting by iteration on a range is equivalent to Delete(range)?
Or there is something other way to accomplish it?
I think Delete(range) can be more efficient as it can perform compaction at same time.