blkdev: Refactoring block io latency histogram codes

The current io_latency_state structure includes entries for read
and write requests. There are special types of write commands
such as sync and discard (trim) commands, and the current
implementation is not general enough if we want to separate
latency histogram for such special commands.

This change makes io_latency_state structure request-type neutral.
It also changes to print the latency average.

Change-Id: Iaa4fbcc428110741b69ed42c02db234e2e2dd5c7
Signed-off-by: Hyojun Kim <hyojun@google.com>
(cherry picked from commit c5a13b37e98e320360e54e13ce50b5b80614cafe)
(cherry picked from commit 66a256326433ebf9f6ec8c9ae8aa54d3fe4fc3b5)
(cherry picked from commit 670c1660c4ddaba56c43cf3320ba039aa397a8f1)
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>