leveldb: rename package 'leveldb/descriptor' to 'leveldb/storage'; this include struct, interface and variable name
Merge remote-tracking branch 'upstream/master'

Conflicts:
	leveldb/bench_test.go
	leveldb/corrupt_test.go
	leveldb/db.go
	leveldb/db_test.go
	leveldb/db_util.go
	leveldb/desc_test.go
	leveldb/session.go
	leveldb/session_util.go
	leveldb/sorted_test.go
	leveldb/table.go
	leveldb/table/reader.go
	leveldb/table/writer.go
	leveldb/util.go
readme: 'descriptor' is now 'storage'