Fix some Helgrind thread errors with enable-tls, and enable-curl.

[philljj]

Fixes a subset of thread errors from PR #395.

Fix some Helgrind thread errors around simultaneous reads and writes from different threads:

• Simultaneous read and write with CURL handle.
• Separate tls.timeout_ms member into separate tls.timeout_ms_read and tls.timeout_ms_write members to prevent simultaneous assignment or overwriting.
• Separate mqttcurl_test_nonblock() into separate read and write functions to fix simultaneous updates from different read/write threads.

This passes whereas PR #395 fails. Apparently the extra locking around SSL struct makes the multithreading CI tests unstable, though it fixes threading errors.

Note: ultimately tested with helgrind and drd:

• valgrind --tool=helgrind --track-lockorders=yes --free-is-write=yes --fair-sched=yes -s
• valgrind --tool=drd --trace-mutex=yes --fair-sched=yes -s

[dgarske]

Jordan, I am assigning this over to @embhorn to review.

[embhorn]

Tested successfully with ./configure --enable-curl --enable-stress --enable-debug=verbose