Persist IV / salt bloom filter into file

[zonyitoo]

shadowsocks/shadowsocks-org#183

Design

• Serialization of PingPongBloom, added serde derive, feature flag and test jedisct1/rust-bloom-filter#19
• sslocal and ssserver allow loading the filter from a specific file
• Persists the current working filter when servers destroy

Problems

1. One server, one bloom filter?
   • The current implementation shares one bloom filter among all servers
2. SIGKILL - Process won't Drop the Context correctly
3. mmap? This will require a full reimplementation of Bloom.

[zonyitoo]

Closing because it is not a effective solution.