Adjust ThreadClient's frames management for nested subsessions

[st0012]

Fixes #563

❯ exe/rdbg -e 'b Foo.bar ;; c ;; bt ;; Foo.bar(10) ;; bt ;; c ;; bt' target.rb
[1, 7] in target.rb
=>   1| class Foo
     2|   def self.bar(num)
     3|     num
     4|   end
     5| end
     6|
     7| binding.b
=>#0    <main> at target.rb:1
(rdbg:commands) b Foo.bar
uninitialized constant Foo

Foo
^^^
#0  BP - Method (pending)  Foo.bar
(rdbg:commands) c
DEBUGGER:  BP - Method  Foo.bar at target.rb:2 is activated.
[2, 7] in target.rb
     2|   def self.bar(num)
     3|     num
     4|   end
     5| end
     6|
=>   7| binding.b
=>#0    <main> at target.rb:7
(rdbg:commands) bt
=>#0    <main> at target.rb:7
(rdbg:commands) Foo.bar(10)
[1, 7] in target.rb
     1| class Foo
     2|   def self.bar(num)
=>   3|     num
     4|   end
     5| end
     6|
     7| binding.b
=>#0    Foo.bar(num=10) at target.rb:3
  #1    <main> at (rdbg)/target.rb:1
  # and 3 frames (use `bt' command for all frames)

Stop by #0  BP - Method  Foo.bar at target.rb:2
(rdbg:commands) bt
=>#0    Foo.bar(num=10) at target.rb:3
  #1    <main> at (rdbg)/target.rb:1
  #2    [C] Binding#eval at ~/projects/debug/lib/debug/thread_client.rb:401
  #3    TracePoint.allow_reentry at <internal:trace_point>:151
  #4    <main> at target.rb:7
(rdbg:commands) c
10
(rdbg:commands) bt
=>#0    <main> at target.rb:7
(rdbg) c    # continue command

[ko1]

fix with #575