obj: make the OBJ_ calls thread safe

obj: make the OBJ_ calls thread safe

This is done using a single global lock.  The premise for this is that new
objects will most frequently be added at start up and never added subsequently.
Thus, the locking will be for read most of the time.

This does, however, introduce the overhead of taking an uncontested read lock
when accessing the object database.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15713)