Convert fts2 to use sqlite3_prepare_v2() to prevent certain logic

Convert fts2 to use sqlite3_prepare_v2() to prevent certain logic
errors around SQLITE_SCHEMA handling.  This also allows
sql_step_statement() and sql_step_leaf_statement() to be replaced with
sqlite3_step().

Also fix a logic error in flushPendingTerms() which was clearing the
term table in case of error.  This was wrong in the face of
SQLITE_SCHEMA.  Even though the change to sqlite3_prepare_v2() should
cause us not to see SQLITE_SCHEMA any longer, it was still a logic
error... (CVS 4205)

FossilOrigin-Name: 16730cb137eaf576b87cdc17913564c9c5c0ed82