caching prep over tdbc

# prep - prepare a stmt or reused an already cached stmt
    method prep {stmt} {
	variable stmts	;# here are some statements we prepared earlier
	if {![info exists stmts]} {
	    set stmts {}
	}
	variable max_prepcache
	if {[dict exists $stmts $stmt]} {
	    set s [dict get $stmts $stmt]
	    if {$max_prepcache > 0} {
		# move matched element to end of cache (for LRU)
		dict unset stmts $stmt
		dict set stmts $stmt $s
	    }
	} else {
	    set s [my db prepare $stmt]
	    dict set stmts $stmt $s
	    if {$max_prepcache > 0 && [dict size $stmts] > $max_prepcache} {
		Debug.session {removing LRU cached statement}
		set stmts [lrange $stmts 2 end]
	    }
	}
	return $s
    }