RFR: 8261941: Use ClassLoader for unregistered classes during -Xshare:dump

[Calvin Cheung]

Before this change, unregistered classes are loaded by the boot class loader during CDS dump time.
This RFE creates an URLClassLoader based on the source specified in the classlist and uses the URLClassLoader to load the unregistered class during CDS dump time. The URLClassLoader instances will be cached in a hash table with the source as the key so that classes associated with the same source will be loaded by the same instance of class loader.

Passed tiers 1 - 4 testing.

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Commit messages:
 - 8261941: Use ClassLoader for unregistered classes during -Xshare:dump

Changes: https://git.openjdk.java.net/jdk/pull/5458/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5458&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261941
  Stats: 115 lines in 7 files changed: 94 ins; 10 del; 11 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5458.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5458/head:pull/5458

PR: https://git.openjdk.java.net/jdk/pull/5458