Minor performance improvements in clients

[tswilliams]

Whilst debugging some uHAL-based software, Andy spotted a couple of minor changes that could improve the performance (and/or reduce CPU usage) of existing clients. In particular:

• The ClientInterface::uri() method currently constructs a new string each time it is called. This method is used in various log messages, with the string-construction penalty incurred whether or not the log message is printed. So, the ClientInterface::uri() method should be updated to return a const reference to a cached URI string.
• In the PCIe::File class, there's a bug in the buffer creation code meaning that as opposed to the intention of the createBuffer method, the buffer is recreated for each transfer.

[tswilliams]

All done (merged into both master and release_2_6, the v2.6.x release branch). Closing.