[php-src] Issue #10854: TSRM should set a smarter value for expected_threads

Issue: https://github.com/php/php-src/issues/10854 Author: nielsdos ### Description The tsrm_startup() function is currently always called with expected_threads = 1. This means that the hashtable used in the TSRM will only contain a single bucket, and all thread resources will therefore be in the same linked list. So it's not really a hashtable atm, even though it's supposed to be. We could improve performance in principle by choosing a more appropriate value for expected_threads, by somehow letting the SAPI give an estimation based on its configuration. I noticed this while working on #10737.