Open TransactionLog for extension

TransactionLog is already extended by HdfsTransactionLog. But the extension basically relies on an empty protected constructor, and HdfsTransactionLog re-implements everything.

We want to open more TransactionLog to extension so that an extending class could leverage the logic present in TransactionLog, and override mainly how output/input streams are opened. This extension will be used by an EncryptionTransactionLog in solr-sandbox.