This change-set:
* Introduces a new top-level directory, mongo/logger, containing most of the implementation of logging functionality formerly found in log.cpp/log.h.
* Cleans up existing, unusual uses of the logging system that were not trivially compatible with the new implementation.
* Replaces Logstream/Nulstream with a LogstreamBuilder object, whose destructor writes log messages. This new LogstreamBuilder is reentrant, unlike the old logging code, which was thread-safe but not reentrant. Additionally, std::endl is no longer required to terminate a log line. When a LogstreamBuilder goes out of scope, the log message gets committed.
* Separates the log system into several components: a global LogManager, several LogDomains, various kinds of Appenders (e.g., SyslogAppender) and Encoders (for formatting messages).
* Allows unit tests to capture and examine log output.
This patch does _not_ introduce support for hierarchical log domains, or for enabling and disabling specific log domains when the server is running in a multi-threaded mode. This is future work.
