Implement a thread-safe Log

[cneumuel]

The current Log implementation is not thread-safe. This is especially problematic if one thread adds a message and another thread (like the UI-thread) enumerates over the log-messages.

Proposal: A ThreadSafeLog which takes another log in the constructor, but ensures thread safety.

ILog threadSafeLog = new ThreadSafeLog(new Log());

[cneumuel]

r6424 added threadsafe log

[mkommend]

Review comments:

• The ThreadSafeLog should implement IDisposable in addition to providing a finalizer (​http://msdn.microsoft.com/en-us/library/fs2xkftw.aspx).
  It is not necessary to implement IDisposeable or a finalizer since the ThreadSafeLog only uses managed resources.
• IMHO the cloning ctor should also request a read lock on the original object.

[mkommend]

r6516: Removed finalizer and locked cloning.

[gkronber]

Reviewed r6516.

[gkronber]

r6536:

• reviewed class ThreadSafeLog
• removed the LockRecursionPolicy.SupportsRecursion option which is not necessary
• added unit test for the thread safe log.

[swagner]

r6547: Changed ThreadSafeLogTest as testing the race condition with the unsafe log resulted in strange exceptions sometimes.