tcpsock:receive needs stream reading pattern

Now tcpsock:receive now support two pattern '_a' and '_l'(default '*l' if pattern is nil), it's quite useful in text-based protocol or open binary protocol(eg. Mysql).

But to deal with non-public protocol, we need stream reading pattern, return anything once socket receives.

For example, we hava a decode library(.so/.dll) privided by others who don't open their protocol, and we need to fill socket data into the library to decode message out. Without steam reading pattern, decode can't be done in efficient way (done by tcpsock:receive(1) loop).

I suggest to add stream reading pattern into tcpsock:receive, especially in stream-lua-nginx-module

Start by locating the tcpsock:receive implementation and its existing _a, _l, and nil-pattern behavior in stream-lua-nginx-module. Compare how socket data is currently returned, then define the stream pattern so data is returned as soon as it is received; done means it supports the non-public protocol use case without a tcpsock:receive(1) loop.