common/io_exerciser: performance, readability and safety improvements

common/io_exerciser: performance, readability and safety improvements

Removes use of const members to allow possibility of moving.
Changes strings that were passed by value to now be const ref.
Adds readability improvements to shorten repeated use of long namespace names in cpp files.
Use std::generator for initialisation of seeds instead of previous for loop

Signed-off-by: Jon Bailey <jonathan.bailey1@ibm.com>