Added proxy_upstream_tries directive, which can limit
proxy_next_upstream retry times. This directive can work in
main/server/http/location config, and is disabled by
default(default behavior is to try ALL upstreams). Partial code is
forked from patch: https://gist.github.com/shai-d/5446961.
Added decode base64 variable
Added md5 variable as a function
Limit proxy/memcached/fastcgi/scgi/uwsgi upstream tries times.
Merge branch 'master' into limit_upstream_tries
Changed the variable's name from md5_ to encode_md5_
fixed coding style
add 'raw_uri' built-in variable for original request URI but without arguments
tiny adjustment
Merge pull request #323 from yaoweibin/coding_style_1

fixed coding style
add 'raw_uri' built-in variable for original request URI but without arguments
add test cases for the new  built-in variable
Merge branch 'master' of github.com:flygoast/tengine
add raw_uri variable assignment in subrequest
fixed a bug of trim module.
add test cases for limit_upstream_tries
disable limit_upstream_tries when xxx_upstream_tries is set to 0.
Merge pull request #322 from supertcy/limit_upstream_tries

Limit proxy/memcached/fastcgi/scgi/uwsgi upstream tries times
nginx-tfs new features and bug-fixs.

1. fix nginx do not close connection bug
2. use readv2 if read from start
3. write failed now retry ns instead of ds
4. fix do not remove dirty block cache when stat large file bug
5. fix pread custom file bug when read offset is larger than 2 times of segment's start offset
6. fix rc timer bug
7. fix write failed bug when data to write is in memory
8. fix select cluster bug
9. add Content-type response header
Merge pull request #329 from zhcn381/upstream2

nginx-tfs new features and bug-fixs.