httpc: fix signed integer overflow

The `lua_add_key_u64()` function pushes an `uint64_t` value using
`lua_pushinteger()`, which accepts `int64_t` argument. A value >= 2^63
will be interpreted as a negative value on all architectures we're
supporting. However, technically it is implementation defined behavior
(see n1256, 6.3.1.3.3).

It is not a problem, in fact, because the function is used only to
report `http_client:stat()` statistics and because values beyond 2^63-1
are unreachable in practice.

OTOH, it is easy to eliminate the undefined behavior by replacing
`lua_pushinteger()` with our own helper function, which accepts
`uint64_t`: `luaL_pushuint64()`.

The values above 10^14 - 1 are now pushed as `cdata<uint64_t>`. Lower
values are pushed as `number` just like before the commit.

Reported-in: https://github.com/tarantool/security/issues/103

NO_DOC=The type of values in the statistics is not specified explicitly
       in the documentation (not obligated to be `number`) and it is
       quite common for Tarantool to return a value of `cdata<int64_t>`
       or `cdata<uint64_t>` type for an integer with a large absolute
       value.
NO_CHANGELOG=see NO_DOC
NO_TEST=It is hard to reach so large values externally (send 2^63
        requests) and it doesn't look worthful to introduce an error
        injection/a internal API to test it. `luaL_pushuint64()` is
        covered by the module API test.

(cherry picked from commit 3dbbf2d3)