util: BufferPool.Put should't do nothing instead of panic'ing after pool been closed
leveldb: fixes compaction cmd that might be stuck if compaction transaction error for extended period
table: Don't embed Reader in the block

The Reader might be released before block does; when this happen the the block will
refer to an invalid Reader instance.

This fixes bug introduced by 546b03ad0e07c6b711ef3f99aa79045bc20194b0 and fixes #72.
Revert "leveldb: Trap for issue #72"

This reverts commit c4f1f5a18485e99337a746c5b21b1e9f5f286a2d.