net.box: allow to store user-defined fields in future object

Before commit 954194a1 ("net.box:
rewrite request implementation in C"), net.box future was a plain Lua
table so that the caller could attach extra information to it. Now it
isn't true anymore - a future is a userdata object, and it doesn't have
indexing methods.

For backward compatibility, let's add __index and __newindex fields and
store user-defined fields in a Lua table, which is created lazily on the
first __newindex invocation. __index falls back on the metatable methods
if a field isn't found in the table.

Follow-up #6241
Closes #6306