vinyl: fix index name in duplicate key error message

The code setting ER_TUPLE_FOUND uses index_name_by_id() to find
the index name, but it passes an index in the dense index map to
it while the function expects an index in the sparse index map.
Apparently, this doesn't work as expected after an index is removed
from the middle of the index map. This bug was introduced by
commit fc3834c0 ("vinyl: check key uniqueness before modifying
tx write set").

Instead of just fixing the index passed to index_name_by_id(), we do
a bit of refactoring. We stop passing index_name and space_name to
vy_check_is_unique_*() functions and instead get them right before
raising ER_TUPLE_FOUND. Note, to get the space name, we need to call
space_by_id() but it should be fine because (a) the space is very likely
to be cached as the last accessed one and (b) this is an error path so
it isn't performance critical. We also drop index_name_by_id() and
extract the index name from the LSM tree object.

Closes #5975

NO_DOC=bug fix

(cherry picked from commit 2cfba5eb)