Feature: upstream dynamic resolve

This is version 1, the "imperfect" version.

Extend "proxy_pass" directive with three new parameters:
*) dynamic_resolve: enables dynamic resolving functionality on an
upstream used by proxy_pass.
*) dynamic_fail_timeout=time, specifies how long tengine will not launch
a dns query if the dns server is down.
*) dynamic_fallback=action, specifies what action to take if the dns
server is considered as "down".

Support:
*) use the first result of a dns response
*) fail_timeout
*) fallback action, the action "stale" is limited to use what tengine gets
when it parses the configuration

Not support:
*) all results from a dns response
*) upstream keepalive
*) ipv6 address
*) using stale dns data
*) ...

All "Not support" stuff are now supported in "perfect" version, which
will be released after tengine is upgraded to original nginx 1.6.x
branch.

Test cases are included.

Signed-off-by: Paul Yang(InfoHunter) <paulyang.inf@gmail.com>
加一行注释吧。这块比较绕