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

This patch increases speed of both hard reset and shutdown by introducing `log.recovery.threads` and `log.shutdown.threads` properties, which allows to perform work required for them in parallel, grained by log directories.

Best performance can be achieved by setting thread count to number of log directories, provided that they are located on dedicated drives. Although that option should be used with caution due to the possibility of native JVM out of memory error.

Patch is compiled of changes proposed by Jay Kreps, Alexey Ozeritskiy, Dmitry Bugaychenko by Anton Karamanov.

All tests are passing.