Change storage to use LevelDB

[hltbra]

I migrated all of the storage to use LevelDB. The results are significantly better and I am ready to try this on a couple of production systems.

I have tried different backends before and they didn't perform better than what we had. I picked LevelDB because it's a small library and a lot simpler than RockDB (fewer features, fewer options, smaller in size). I am definitely considering RocksDB if we think the performance can be better with the usage of column families instead of one keyspace with every value in it.

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With this pull request, there's one LevelDB database per Redis database (16 total).
Each data structure has at least one key on LevelDB:

type	number of keys in leveldb
string	1
hash	1 + 1 key per field
set	1 + 1 key per element
sorted set	1 + 1 per score + 1 per element

type	data it stores
string	key value
hash	number of elements in the hash
hash field	value of the hash field
set	number of elements in the set
set element	nothing
sorted set	number of elements in the sorted set
sorted set element	the score of the element
sorted set score	nothing

All LevelDB keys have prefixes indicating their type (e.g., hash field, set member, sorted set score). The prefixes were inspired by the blackwidow project ^{1 2}

Examples:

>>> from dredis.ldb import LEVELDB
>>> from dredis.keyspace import Keyspace
>>> LEVELDB.setup_dbs('/tmp/tests')
>>> keyspace = Keyspace()
>>> keyspace.set('mystring', 'stringvalue')
>>> keyspace.hset('myhash', 'field1', 'value1')
1
>>> keyspace.sadd('myset', 'member1')
1
>>> keyspace.zadd('mysortedset', 5, 'element1')
1
>>> keyspace.zadd('mysortedset', 10, 'element2')
1
>>> for db_key, db_value in LEVELDB.get_db('0'):
...     print('%r = %r' % (db_key, db_value))
... 
'\x01\x00\x00\x00\x08mystring' = 'stringvalue'
'\x02\x00\x00\x00\x05myset' = '1'
'\x03\x00\x00\x00\x05mysetmember1' = ''
'\x04\x00\x00\x00\x06myhash' = '1'
'\x05\x00\x00\x00\x06myhashfield1' = 'value1'
'\x06\x00\x00\x00\x0bmysortedset' = '2'
'\x07\x00\x00\x00\x0bmysortedsetelement1' = '5'
'\x07\x00\x00\x00\x0bmysortedsetelement2' = '10'
'\x08\x00\x00\x00\x0bmysortedset@\x14\x00\x00\x00\x00\x00\x00element1' = ''
'\x08\x00\x00\x00\x0bmysortedset@$\x00\x00\x00\x00\x00\x00element2' = ''

[jimjshields]

🎆