vinyl: use xalloc for vylog allocations

The vinyl metadata log processor allocates its internal objects either
from malloc or region, neither of which should fail for small
allocations. Let's switch to xalloc to simplify the code. A good thing
about this change is that now we can ignore all errors raised by
vy_log_record_decode() and vy_recovery_process_record() if the
force_recovery flag is set (see the next commit).

Needed for #10292

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring

(cherry picked from commit b6f015e98b7c30861dc33032ba7eca47de0cc198)