sheepfs: add framework for http interface

Adding http.c in sheepfs dir as the framework and implement "address" file
to store http address.

Signed-off-by: Robin Dong <sanbai@taobao.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
sheepfs: implement the http interface of sheepfs by using libcurl

Use libcurl to call the interface of http for object-storage and implement
the read/write operation.

Signed-off-by: Robin Dong <sanbai@taobao.com>
[ replace sd_err by sheepfs_pr ]
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
test: add test-case for http interface of sheepfs

Signed-off-by: Robin Dong <sanbai@taobao.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>