Using start==0 to indicate a stopped timer is dangerous, because 0 is a
valid value for the current tick counter.

New min_timeout and max_timeout fields in struct retry_timer allow
users of this timer to set their own desired minimum and maximum
timeouts, without being constrained to a single global minimum and
maximum. Users of the timer can still elect to use the default global
values by leaving the min_timeout and max_timeout fields as 0.

Allow for timers to be started with fixed timeouts.

Signed-off-by: Alexey Zaytsev <alexey.zaytsev@gmail.com>

permanent processes, rather than requiring each one to have its own
initialisation function.

Add reference-counting to processes.

Add timer_running() test.

defined in vsprintf.h.  (This may change, since vsprintf.h is a
non-standard name, but for now it's the one to use.)

There should be no need to include vsprintf.h just for DBG() statements,
since include/compiler.h forces it in for a debug build anyway.

AoE now uses the retry timer mechanism.

these timer objects in AoE and UDP protocols (where there is no underlying
retransmission mechanism) without requiring each protocol to implement its
own individual retry logic.  Eventually, we should be able to use the same
timer code for TCP retransmissions as well.