Avoid compiling DARs more than necessary in tests

Fixes #169.

• DamlSdk.buildDar now memoizes compilation for the whole functest run via Docker.share, keyed by the DamlSource and the LF target.
• Daml sources duplicated byte-for-byte across specs move into Dars
• Four single-template Ping variants become one Dars.pingPong, so each is compiled once.

• Matrix run recommended: FlywayMigrationSpec's 3.4 and 3.5 upgrade cases (LF ≤ 2.2 and LF 2.3) now deploy Dars.pingPong (Ping(sender), transact1) and run only in the matrix.