ffi: use a new C library namespace when the dynamic library load

If symbols from the dynamic library are loaded into the
global namespace (ffi.C), by invoke ffi.load() with global
is true and without save the return value, it may be
garbage collected. For correct work the return value
must be saved.
Example - tarantool/tarantool#4999
So, use a new C library namespace instead the global one.
This will help to keep the global namespace clean and
avoid the problem with garbage collection.

memcached/init.lua

@@ -10,7 +10,7 @@ local fmt = string.format
 
 local internal_so_path = package.search('memcached.internal')
 assert(internal_so_path, "Failed to find memcached/internal.so library")
-ffi.load(internal_so_path, true)
+local memcached_internal = ffi.load(internal_so_path)
 
 ffi.cdef[[
 struct memcached_stat {
@@ -267,7 +267,8 @@ local memcached_methods = {
         end
         for k, v in pairs(opts) do
             if conf_table[k] ~= nil then
-                C.memcached_set_opt(self.service, conf_table[k], v)
+                memcached_internal.memcached_set_opt(self.service,
+                                                     conf_table[k], v)
                 self.opts[k] = v
             end
         end
@@ -276,14 +277,14 @@ local memcached_methods = {
     start = function (self)
         local function memcached_handler(socket, addr)
             log.debug('client %s:%s connected', addr.host, addr.port)
-            C.memcached_handler(self.service, socket:fd())
+            memcached_internal.memcached_handler(self.service, socket:fd())
         end
         jit.off(memcached_handler)
 
         if self.status == RUNNING then
             error(fmt(err_is_started, self.name))
         end
-        C.memcached_start(self.service)
+        memcached_internal.memcached_start(self.service)
         local parsed = uri.parse(self.uri)
         self.listener = socket.tcp_server(parsed.host, parsed.service, {
             handler = memcached_handler
@@ -293,7 +294,7 @@ local memcached_methods = {
             self.status = ERRORED
             error(fmt('can\'t bind (%d) %s', errno(), errno.strerror()))
         end
-        if (C.memcached_setsockopt(self.listener:fd(),
+        if (memcached_internal.memcached_setsockopt(self.listener:fd(),
                                    lname.family,
                                    lname.type) == -1) then
             self.status = ERRORED
@@ -309,12 +310,12 @@ local memcached_methods = {
         if (self.listener ~= nil) then
             self.listener:close()
         end
-        local rc = C.memcached_stop(self.service)
+        local rc = memcached_internal.memcached_stop(self.service)
         self.status = STOPPED
         return self
     end,
     info = function (self)
-        local stats = C.memcached_get_stat(self.service)
+        local stats = memcached_internal.memcached_get_stat(self.service)
         local retval = {}
         for k, v in pairs(stat_table) do
             retval[v] = stats[0][v]
@@ -362,11 +363,12 @@ local function memcached_init(name, uri, opts)
     else
         instance.space = box.space[instance.space_name]
     end
-    local service = C.memcached_create(instance.name, instance.space.id)
+    local service = memcached_internal.memcached_create(instance.name,
+                                                        instance.space.id)
     if service == nil then
         error(fmt(err_enomem, "memcached service"))
     end
-    instance.service = ffi.gc(service, C.memcached_free)
+    instance.service = ffi.gc(service, memcached_internal.memcached_free)
     memcached_services[instance.name] = setmetatable(instance, {
         __index = memcached_methods
     })