KAFKA-1414: Speedup broker shutdown and startup after hard reset

This patch increases speed of both hard reset and shutdown by introducing `log.io.parallelism` property, which specifies the amount of threads to be used when performing io intensive work on logs.

Note that those options should be used with caution due to the possibility of native JVM out of memory error when using a lot of threads.

Patch is compiled of changes proposed by Alexey Ozeritskiy, Dmitry Bugaychenko by Anton Karamanov.

All tests are passing.