accounthist: provide a `tools` to do manual purging of old keys

[abourget]

Manual-run clean-up process

After all sharded loading is done, we could go through all accounts by doing:

SCAN 02:0000000000000 02:fffffffffffffff LIMIT 1
 -> 02:000000b1:ff:fffffff0000 -> 65535 actions
    over 1000 (maxEntries), so:
    SCAN 02:0000000b1:00:00000000000000  02:0000000b1:fe:fffffffffffff
    * Get all those keys and delete them
SCAN: 02:000000000b2 02:FFFFFFFFFFFFFFF LIMIT 1  (that's the next account, b2)
 -> 02:00000eosio:ff:fffffffffff8
    got only 10, let's continue on other shards (shardNum = 0 here), to get
    to a SUM of 1000, THEN start deleting older keys.

etc..