search: make xap_terms easier-to-use and use it more

search: make xap_terms easier-to-use and use it more

This allows us to simplify callers throughout, and exceptions are
can no longer be silently hidden.  MiscSearch now uses xap_terms
for looking up eidx_key terms for a code reduction.

We also simplify LeiStore->_msg_kw for runtime use by moving the
MsetIterator handling into t/lei_store.t test case.