Bump version of small library

[ligurio]

Now we are stick to small submodule with the latest commit made on Jan 2020:

commit 3df5050171d040bfe5717b4bddf57da3b312ffe4 (HEAD)
Author: Alexander Turenko <[email protected]>
Date:   Tue Jan 28 16:35:26 2020 +0300

    Revert "Free all slabs on region reset"

Changes added since Jan 2020: https://github.com/tarantool/small/compare/3df5050171d040bfe5717b4bddf57da3b312ffe4..master

Related to #59
Depends on #96

[Totktonada]

We can't just bump the library, because we'll likely break compatibility with tarantool 2.5-2.8. See #59 (comment) for details. We have the following options:

• Wait until tarantool 2.5-2.8 will gone and forgotten (a year after 2.10.0 release at least). Update the library in the module together with a version check in the module, which will raise an error from memcached.create() on tarantool 2.5-2.8.
• Copy small's code into the module and rename functions (and globals if any) to avoid possible name clash. It seems, we should rename everything related to mempool, region, ibuf, obuf and everything that is used inside them. Only functions from *.c files or inline functions as well? Don't know.
• Investigate, whether -Wl,-Bsymbolic-functions allows to avoid the name clash (see Segfault if used with tarantool debug build #59 (comment)).
• Choose symbols in runtime (dlopen() with RTLD_DEEPBIND plus dlsym()) similarly how the tarantool/smtp module do (see again Segfault if used with tarantool debug build #59 (comment)).

I like the first option, because it means just "don't do anything" :)