vinyl: move cache invalidation from vy_tx_write to vy_lsm_set

vy_lsm.c seems to be a more appropriate place for cache invalidation
because (a) it's vy_lsm that owns the cache and (b) we invalidate
the cache on rollback in vy_lsm_rollback_stmt().

While we are at it, let's inline vy_tx_write() and vy_tx_write_prepare()
because they are trivial and used in just one place.

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring

(cherry picked from commit 44c245ef0baa227a6bff75de2a91f20da5532dc1)