Rename PAGE_SIZE to ENGINE_PAGE_SIZE to avoid a clash
Fixes the build with musl libc because it clashes with PAGE_SIZE from limits.h, which according to POSIX [1] should be in there. Fails with "KNSBackend.cpp:39:18: error: expected unqualified-id before numeric constant 39 | static const int PAGE_SIZE = 100;" otherwise.
[1] https://pubs.opengroup.org/onlinepubs/009696899/basedefs/limits.h.html