xrow: remove SYNCHRO_BODY_LEN_MAX constant

Synchronous requests will receive a new field encoding a full vclock
soon. Theoretically a vclock may take up to ~ 300-400 bytes (3 bytes for
a map header + 32 components each taking up 1 byte for replica id and up
to 9 bytes for lsn). So it makes no sense to increase
SYNCHRO_BODY_LEN_MAX from 32 to 400-500. It would become almost the same
as plain BODY_LEN_MAX. Simply reuse the latter everywhere.

In-scope-of #9138

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring

(cherry picked from commit 53605779)