Indent SIGINFO thread backtraces (#2071)

Before this commit the SIGINFO thread backtraces looked like:

```
Thread: TID-owjccjjvw puma reactor
/path/to/puma/reactor.rb:136 in 'select'
/path/to/puma/reactor.rb:136 in 'run_internal'
/path/to/puma/reactor.rb:313 in 'run_in_thread'
Thread: TID-owjccjjrg puma threadpool reaper
/path/to/puma/thread_pool.rb:262 in 'sleep'
/path/to/puma/thread_pool.rb:262 in 'block in start!'
```

This format can make it a bit difficult to see where one backtrace ends
and the next begins. This commit indents the backtrace lines for clearer
visual separation.

```
Thread: TID-owjccjjvw puma reactor
  /path/to/puma/reactor.rb:136 in 'select'
  /path/to/puma/reactor.rb:136 in 'run_internal'
  /path/to/puma/reactor.rb:313 in 'run_in_thread'
Thread: TID-owjccjjrg puma threadpool reaper
  /path/to/puma/thread_pool.rb:262 in 'sleep'
  /path/to/puma/thread_pool.rb:262 in 'block in start!'
```

lib/puma/launcher.rb

@@ -455,7 +455,7 @@ module Puma
         Signal.trap "SIGINFO" do
           thread_status do |name, backtrace|
             @events.log name
-            @events.log backtrace
+            @events.log backtrace.map { |bt| "  #{bt}" }
           end
         end
       rescue Exception