Fix set_alloc_hint op cause KeyValueStore crash problem

Now KeyValueStore doesn't support set_alloc_hit op, the implementation of
_do_transaction need to consider decoding the arguments. Otherwise, the
arguments will be regarded as the next op.

Fix the same problem for MemStore.

Fix #8381

Reported-by: Xinxin Shu <xinxin.shu5040@gmail.com>
Signed-off-by: Haomai Wang <haomaiwang@gmail.com>
Merge pull request #1840 from yuyuyu101/wip-8381

Fix set_alloc_hint op cause KeyValueStore crash problem

Reviewed-by: Samuel Just <sam.just@inktank.com>