The SSL_Session object is mutated during connection inside openssl,
and it might not be thread-safe. Besides, according to documentation
of openssl:
```
SSL_SESSION objects keep internal link information about the session
cache list, when being inserted into one SSL_CTX object's session
cache. One SSL_SESSION object, regardless of its reference count,
must therefore only be used with one SSL_CTX object (and the SSL
objects created from this SSL_CTX object).
```
If I understand correctly, it is not safe to share it even in a
single thread.
Instead, serialize the SSL_SESSION before adding it to the cache,
and deserialize it after retrieving it from the cache, so that no
concurrent write to the same object is infeasible.
Also
- add a ci test for thread sanitizer
- add a test for sharing ssl sessions concurrently
- avoid redefining memory functions when not building libcurl, but
including the soruce in libtest
- increase the concurrent connections limit in sws
Notice that there are fix for a global data race for openssl which
is not yet release. The fix is cherry pick for the ci test with
thread sanitizer.
d8def79838Closes#14751
