Basic Lua scripting support

[badrishc]

This PR adds very basic support for Lua scripts in Garnet (EVAL, EVALSHA, SCRIPT commands) using the NLua library. We hope for contributions from the community to extend as per their needs. Consider this a research prototype. Note that performance is not expected to be as good as C# custom procedures and transactions, due to the overheads of Lua scripts and the conversions from C# to Lua and back.

Tasks:

• Basic tests passing for GET, SET, ZADD
• BDN tests to see performance limits when using NLua
• Optimize script invocation code path for end-to-end performance
• Add support for all API calls in Lua scripts (by formatting into RESP command)
• Error handling, support more types, math
• Add switch (--lua and EnableLua) to enable Lua script on server (turned off by default)
• Enable transactions (opt in at server-level) via --lua-transaction-mode and LuaTransactionMode boolean option
• Support ACL

Future work:

• Better script cache management (currently all scripts are cached with no upper bound)
• Support LuaTransactionMode on a per-script basis
• Support shared vs exclusive key locks (currently we pessimistically lock all keys as exclusive)
• Add cluster slot verification
• Support read-only scripts (EVAL_RO)

Fix #509

[badrishc]

End-to-End Performance

Increasing number of client sessions, 8-byte keys and values, using Resp.benchmark with batch size 4096, no transactions.

1. Script: return KEYS[1]

How to run: Resp.benchmark --op SCRIPTRETKEY -t 1,2,4,8,16 --dbsize 1024

2. Script: return redis.call('get', KEYS[1])

How to run: Resp.benchmark --op SCRIPTGET -t 1,2,4,8,16 --dbsize 1024