Fix for NetworkParameters construction thread safety. Resolves issue #549.

Breaks dependency cycle between Script.\<clinit>, Script.\<init> and NetworkParamenters.\<init>.

Script.\<clinit> depended on both 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 hopefully removes the root cause behind NetworkParameters construction not being thread safe.