Last night I heard the latest incarnation of the Juilliard String Quartet in recital at Alice Tully Hall. The program included performances of Mendelssohn’s first String Quartet and the juggernaut that is Beethoven’s Op. 130 with the Grosse Fuge finale, both pieces performed with suavity rather than abundant risk-taking. The highlight was the quartet’s New York premiere of Mario Davidovsky’s Sixth String Quartet, “Fragments.”  Davidovsky’s description of the quartet is accurate in that it includes fragments of motivic material that are juxtaposed in a variety of ways. However, it is anything but fragmentary in terms of the consistent feeling of a long line’s presence and persistent through