Fix to the PrivateKeys example. Thanks to recallfx for the patch.
Breaks dependency cycle between Script.<clinit>, Script.<init> and NetworkParamenters.<init>. Resolves issue #549.

Script.<clinit> depended both on Script.<init> and NetworkParameters.<init> to prepare ScriptChunk cache.

At the same time NetworkParameters.<init> depended on Script.<clinit> by using Script.writeBytes().

The fix frees Script.<clinit> from both the NetworkParameters and Script objects.

This should remove the root cause behind NetworkParameters construction not being thread safe.
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