Adds multiple process support to the BufferedWriter class

[rprevot]

This pull request adds multiple process support to the BufferedWriter class.

Without this patch, enabling I/O buffering for a file appender generates an error at application startup when log4cxx has been compiled with the multi-process flag set (-DCMAKE_CXX_FLAGS="-DLOG4CXX_MULTI_PROCESS") :

terminate called after throwing an instance of 'std::logic_error'
what(): getOutPutStreamPtr must be implemented in the derived class that you are using
Aborted (core dumped)

Example of configuration:

log4j.appender.State=RollingFileAppender
log4j.appender.State.layout=PatternLayout
log4j.appender.State.layout.ConversionPattern=%d{yyyy-MM-dd HH:mm:ss,SSS Z} %t [%p] %m%n
log4j.appender.State.file=${logdir}/state.log
log4j.appender.State.bufferedIO=true
log4j.appender.State.bufferSize=8192
...