Change default value for BufferPageAllocSize

[abyss7]

Current value 128K potentially leads to OOM in KQP_SCAN_COMPUTE_ACTORs on multiple prod databases.

Change default value to 4K - investigate better value for future releases through perf-tests.
Anyway, there is no proof that the value of this option is a cause of OOMs - the root cause may be somewhere else.

[abyss7]

Perf-test results: https://nda.ya.ru/t/xaXdm-dK7MZQv7