add precompiled contracts for sharded storage

[qizhou]

This is a significant PR that supports sharded storage via precompiled contracts. The major changes from the PR are

• Add 0x33301 precompiled contract to deploy a system contract at a specific address.
• Add 0x33302 precompiled contract to offer raw put/get operations to the underlying storage.
• Add 0x3330001 system contract deployed using DecentralizedKV contract. This one is temporary and we will upgrade it to minable contract soon.
• Add interfaces to StateDB to handle basic sharded storage read/write/commit (thread unsafe)
• Add interfaces to trie.Database to handle basic sharded storage read/write/commit (thread safe)

How to test it?

• Run sstorage under cmd/sstorage like ./sstorage create --filename ~/ss0.dat --kv_size=4096 --len=262144. This will create a data file containing 262144 entries (1G per shard at the moment)
• Run geth with an additional command like --sstorage.shard=4k,0 --sstorage.file=4k,/Users/qizhou/ss0.dat to tell that the node will be responsible for shard 0 and the location of the data file.
• Use remix and use compiled DecentralizedKV at 0x3330001. Then you could test put/get for large storage via remix.

Suggested code review path:

• contracts.go (precompiled contracts)
• statedb.go
• trie/database.go
• other files

[qizhou]

StateDB snapshot test flow:

1. Add a putRawWithRevert() method in DecentralizedKV, which will revert after putRaw()
2. Before the snapshot support: after calling putRawWithRevert(), observed the sharded storage is changed even the tx is failed
3. After the snapshot support: after calling putRawWithRevert(), observed the sharded storage is unchanged even the tx is failed