Modernize and improve the timer module

Remove most of the timer management overhead from the single
timer server process, and decentralize the actual process of
timing, to make the timer server more efficient and less prone
to becoming overloaded. The outward behavior of the timer
module is unchanged.

- Start the timer server under kernel_sup instead of
  kernel_safe_sup.

- Move argument validations out of the timer server and
  into the API functions.

- One-shot send timers are using erlang:send_after and
  erlang:cancel_timer in the client process, without going
  through the timer server, when the destination is a pid
  on the local node.

- The timer server does not do any actual timing any more but
  only executes actions and reschedules interval timers.
  Instead, erlang:start_timer is used for the timing.

- The timer table is not named any more, an ets:tid()
  is carried in a state variable instead.

- Processes related to interval timers are monitored (vs linked)
  by the timer server in order to cancel them when the related
  process dies.

- timer:sleep/1 now accepts arbitrarily high integer values.
  Integer values above the limit of what is allowed in the after
  clause of a receive expression are split up.

- Document that send_after and send_interval accept {RegName, Node}
  as message destinations.

- The function get_status/0 that was only used in tests has been removed,
  the tests now use sys:get_state instead.

- Test return values.

lib/stdlib/doc/src/timer.xml

@@ -50,7 +50,9 @@
     <p>Creating timers using
       <seemfa marker="erts:erlang#send_after/3">erlang:send_after/3</seemfa> and
       <seemfa marker="erts:erlang#start_timer/3">erlang:start_timer/3</seemfa>
-      is much more efficient than using the timers provided by this module. See
+      is more efficient than using the timers provided by this module. However,
+      the timer module has been improved in OTP 25, making it more efficient and
+      less susceptible to being overloaded. See
       <seeguide marker="system/efficiency_guide:commoncaveats#timer-module">the
       Timer Module section in the Efficiency Guide</seeguide>.</p>
   </description>
@@ -116,8 +118,9 @@
           <c>exit_after(<anno>Time</anno>, self(),
           <anno>Reason1</anno>)</c>.</p>
         <p><c>exit_after/3</c> sends an exit signal with reason
-          <c><anno>Reason1</anno></c> to
-          pid <c><anno>Pid</anno></c>. Returns <c>{ok, <anno>TRef</anno>}</c>
+          <c><anno>Reason1</anno></c> to <c><anno>Target</anno></c>,
+          which can be a local process identifier or an atom of a registered
+          name. Returns <c>{ok, <anno>TRef</anno>}</c>
           or <c>{error, <anno>Reason2</anno>}</c>.</p>
       </desc>
     </func>
@@ -149,7 +152,7 @@
         <p><c>kill_after/1</c> is the same as
           <c>exit_after(<anno>Time</anno>, self(), kill)</c>.</p>
         <p><c>kill_after/2</c> is the same as
-          <c>exit_after(<anno>Time</anno>, <anno>Pid</anno>, kill)</c>.</p>
+          <c>exit_after(<anno>Time</anno>, <anno>Target</anno>, kill)</c>.</p>
       </desc>
     </func>
 
@@ -190,15 +193,16 @@
     <func>
       <name name="send_after" arity="2" since=""/>
       <name name="send_after" arity="3" since=""/>
-      <fsummary>Send <c>Message</c> to <c>Pid</c> after a specified
+      <fsummary>Send <c>Message</c> to <c>Destination</c> after a specified
         <c>Time</c>.</fsummary>
       <desc>
         <taglist>
           <tag><c>send_after/3</c></tag>
           <item>
-            <p>Evaluates <c><anno>Pid</anno> ! <anno>Message</anno></c> after
-              <c><anno>Time</anno></c> milliseconds. (<c><anno>Pid</anno></c>
-              can also be an atom of a registered name.)</p>
+            <p>Evaluates <c><anno>Destination</anno> ! <anno>Message</anno></c> after
+              <c><anno>Time</anno></c> milliseconds. (<c><anno>Destination</anno></c> can
+              be a remote or local process identifier, an atom of a registered name or
+              a tuple <c>{RegName, Node}</c> for a registered name at another node.)</p>
             <p>Returns <c>{ok, <anno>TRef</anno>}</c> or
               <c>{error, <anno>Reason</anno>}</c>.</p>
             <p>See also
@@ -223,10 +227,11 @@
         <taglist>
           <tag><c>send_interval/3</c></tag>
           <item>
-            <p>Evaluates <c><anno>Pid</anno> ! <anno>Message</anno></c>
+            <p>Evaluates <c><anno>Destination</anno> ! <anno>Message</anno></c>
               repeatedly after <c><anno>Time</anno></c> milliseconds.
-              (<c><anno>Pid</anno></c> can also be
-              an atom of a registered name.)</p>
+              (<c><anno>Destination</anno></c> can be a remote or local process
+              identifier, an atom of a registered name or a tuple <c>{RegName, Node}</c>
+              for a registered name at another node.)</p>
             <p>Returns <c>{ok, <anno>TRef</anno>}</c> or
               <c>{error, <anno>Reason</anno>}</c>.</p>
           </item>
@@ -249,6 +254,11 @@
           or suspends the process forever if <c><anno>Time</anno></c> is the
           atom <c>infinity</c>. Naturally, this
           function does <em>not</em> return immediately.</p>
+        <note>
+          <p>Before OTP 25, <c>timer:sleep/1</c> did not accept integer
+            timeout values greater than <c>16#ffffffff</c>, that is, <c>2^32-1</c>.
+            Since OTP 25, arbitrarily high integer values are accepted.</p>
+        </note>
       </desc>
     </func>
 
@@ -350,4 +360,3 @@ timer:cancel(R),
       <seemfa marker="#cancel/1"><c>cancel/1</c></seemfa>.</p>
   </section>
 </erlref>
-