Transaction Log - Refactor out Applier from Log

Currently, the transaction log module implements the plugin::TransactionApplier interface in a class called TransactionLog. Because of the new XA work and future designs for multiple log files acting in a rotation pair, it is necessary to split the applier interface from the TransactionLog class.

Steps to take:

1) Make the TransactionLog class a simple descriptor for the actual transaction log file
2) Split out the TransactionLogApplier into separate class with constructor taking a TransactionLog instance
3) Split the module initialization stuff out into a file, /plugin/transaction_log/module.cc