Optimize memory consumption in ResultCacheManager

[staabm]

reproduced with

php bin/phpstan clear-result-cache -q && blackfire run --ignore-exit-status php -d memory_limit=448M bin/phpstan

the master process had a memory spike in sprintf() calls when writing the result cache

before this PR: (1.10.x@7c4ef6b2a)

Wall Time       35s
I/O Wait      19.2s
CPU Time      15.7s
Memory        221MB
Network         n/a     n/a     n/a
SQL             n/a     n/a

---

after this PR:

CPU Time      14.4s
Memory        197MB
Wall Time       33s
I/O Wait      18.7s
Network         n/a     n/a     n/a
SQL             n/a     n/a

[phpstan-bot]

You've opened the pull request against the latest branch 1.11.x. If your code is relevant on 1.10.x and you want it to be released sooner, please rebase your pull request and change its target to 1.10.x.

[ondrejmirtes]

Thank you!