[lldb] [gdb-remote client] Refactor SetCurrentThread*()

Refactor SetCurrentThread() and SetCurrentThreadForRun() to reduce code
duplication and simplify it.  Both methods now call common
SendSetCurrentThreadPacket() that implements the common protocol
exchange part (the only variable is sending `Hg` vs `Hc`) and returns
the selected TID.  The logic is rewritten to use a StreamString
instead of snprintf().

A side effect of the change is that thread-id sent is now zero-padded.
However, this should not have practical impact on the server as both
forms are equivalent.

Differential Revision: https://reviews.llvm.org/D100459