Cellular: Make AT_CellularStack socket array multi-thread safe

[AriParkkila]

Description

The socket array _sockets was not multi-thread safe. That could be a problem if two threads try to open socket at the same.

In socket_close() was delete socket called erroneously before calling socket_close_impl, that may still use that socket.

Pull request type

[X] Fix
[ ] Refactor
[ ] Target update
[ ] Functionality change
[ ] Breaking change

[AriParkkila]

@mirelachirica please review

[jarvte]

Declaration of _socket_mutex is ifdef'd but not it's usage. Remove #ifdef or add them also where it's used.

[AriParkkila]

@jarvte please review

[0xc0170]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 2916
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7857/

Triggering tests

/morph test
/morph uvisor-test
/morph export-build
/morph mbed2-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2535
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7857/

[mbed-ci]

Test : SUCCESS

Build number : 2665
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7857/2665