This PR adds `DeleteRange` to `ethdb.KeyValueWriter`. While range
deletion using an iterator can be really slow, `DeleteRange` is natively
supported by pebble and apparently runs in O(1) time (typically 20-30ms
in my tests for removing hundreds of millions of keys and gigabytes of
data). For leveldb and memorydb an iterator based fallback is
implemented. Note that since the iterator method can be slow and a
database function should not unexpectedly block for a very long time,
the number of deleted keys is limited at 10000 which should ensure that
it does not block for more than a second. ErrTooManyKeys is returned if
the range has only been partially deleted. In this case the caller can
repeat the call until it finally succeeds.
Felföldi Zsolt
80bdab757d