Summary:
After move assignment, we need to re-initialized the moved PinnableSlice.

Also update blob_db_impl.cc to not reuse the moved PinnableSlice since it is supposed to be in an undefined state after move.
Closes https://github.com/facebook/rocksdb/pull/3127

Differential Revision: D6238585

Pulled By: yiwu-arbug

fbshipit-source-id: bd99f2e37406c4f7de160c7dee6a2e8126bc224e