connpool: move source code file

The source files for built-in Lua modules are generally placed on the
same level: in `src/lua` or in `src/box/lua`, disregarding whether
they're public or internal.

The recently introduced `experimental.connpool` built-in module is
placed in the `experimental` subdirectory.

This commit moves `src/box/lua/experimental/connpool.lua` to
`src/box/lua/connpool.lua` to follow the existing file structure.
Public, internal and experimental modules are all on the same level now.

The `connpool` module is still experimental and
`require('experimental.connpool')` is needed to use it.

This commit doesn't change the code of the module.

NO_DOC=no code changes
NO_CHANGELOG=see NO_DOC
NO_TEST=see NO_DOC