core: improve performance

[maxux]

There are components written to get quickly a working filesystem, but there are a lot of improvements possible:

• Profil code to see where bottlenecks are
• Improve how search are made, for now any lookup are linear search
• Ensure cache won't loose data
• Improve code to not crash on backend error
• Try to reconnect on backend down
• ...

[maxux]

• [x] Lookup are now using binary search
• [x] Cache lookup and using hash-prefix

[maxux]

Overall performance are increased by around 10% by using unix socket in place of tcp to zdb.