scylla-gdb.py: Introduce "scylla thread" command
scylla-gdb.py: Introduce "scylla thread" command

Switches into a seastar thread. "scylla unthread" switches back.

Example:

```
  (gdb) scylla thread 0x6000000e1800
  Switched to thread 1, (seastar::thread_context*) 0x6000000e1800

  (gdb) bt
  #0  seastar::thread_context::switch_out (this=0x6000000e1800) at core/thread.cc:104
  #1  0x00000000004cfb07 in future<>::wait() (this=0x600008ca2c70) at core/future.hh:817
  #2  0x0000000000f7752c in future<>::get() (this=0x600008ca2c70) at /home/tgrabiec/src/scylla2/seastar/core/future.hh:787
  ...
  #16 seastar::thread_context::main (this=0x6000000e1800) at core/thread.cc:166
  #17 0x000000000051a702 in seastar::thread_context::s_main (lo=<optimized out>, hi=<optimized out>) at core/thread.cc:157
  #18 0x00007f2c34861f20 in ?? () from /lib64/libc.so.6
  #19 0x0000000000000000 in ?? ()

  (gdp) scylla unthread
  Switched to thread 1
```

raphaelsc/scylla,kjniemi/scylla,avikivity/scylla,scylladb/scylla,raphaelsc/scylla,duarten/scylla,scylladb/scylla,kjniemi/scylla,raphaelsc/scylla,scylladb/scylla,duarten/scylla,kjniemi/scylla,scylladb/scylla,avikivity/scylla,duarten/scylla,avikivity/scylla