options: allow separate values for max_latency

options: allow separate values for max_latency

Adding the ability to specify max_latency limits separately for read, write, trim.
Uses comma-separated values similar to blocksize, rate parameters etc.

Also modifies the behavior for the one option only case for max_latency and latency_target
by excluding syncs. This makes the latency limits similar to the rate checks, in that they
only apply to reads, writes and trims.

Extends the output printed on latency exceeded event, to display io_unit information using format
similar to that iolog write.

Signed-off-by: Venkat Ramesh <venkatraghavan@fb.com>