ASSERT dcontext == get_thread_private_dcontext() in dr_read_saved_reg is too strong

From the users list https://groups.google.com/forum/#!topic/DynamoRIO-Users/Q_aC_MogiGg

> I'm trying to use a tool to extract some data on an MPI application that uses some asynchronous communication, but I'm getting the following error:
>
> parent 0x000000004b449d70, 0x00005574ec406592 report_dynamorio_problem
frame ptr 0x000000004b449d70 => parent 0x000000004b449dc0, 0x00005574ec400e17 external_error
frame ptr 0x000000004b449dc0 => parent 0x000000004b449df0, 0x00005574ec506ac3 dr_read_saved_reg
frame ptr 0x000000004b449df0 => parent 0x000000004b449e20, 0x0000000078002ace get_spilled_value
frame ptr 0x000000004b449e20 => parent 0x000000004b449f70, 0x00000000780064bf drreg_event_restore_state
frame ptr 0x000000004b449f70 => parent 0x000000004b44a1d0, 0x000000007300540a drmgr_restore_state_event
frame ptr 0x000000004b44a1d0 => parent 0x000000004b44a270, 0x00005574ec4fe52e instrument_restore_state
frame ptr 0x000000004b44a270 => parent 0x000000004b44a8b0, 0x00005574ec51120e recreate_app_state_internal
frame ptr 0x000000004b44a8b0 => parent 0x000000004b44a8f0, 0x00005574ec5115e0 recreate_app_state
frame ptr 0x000000004b44a8f0 => parent 0x000000004b44a940, 0x00005574ec4ec421 at_safe_spot
frame ptr 0x000000004b44a940 => parent 0x000000004b44ac60, 0x00005574ec4ed878 synch_with_thread
frame ptr 0x000000004b44ac60 => parent 0x000000004b44ad20, 0x00005574ec4eeee3 synch_with_all_threads
frame ptr 0x000000004b44ad20 => parent 0x000000004b44ada0, 0x00005574ec3d2f47 fcache_reset_all_caches_proactively
frame ptr 0x000000004b44ada0 => parent 0x000000004b44ae30, 0x00005574ec39be72 enter_nolinking
frame ptr 0x000000004b44ae30 => parent 0x000000004b44af20, 0x00005574ec3ea4c2 dispatch_enter_fcache
frame ptr 0x000000004b44af20 => parent 0x000000004b44aff0, 0x00005574ec3e9e47 dispatch
```

That assert is too strong as this is a legitimate use case within DR. There is a similar issue in dr_write_saved_reg.

Start in core/lib/instrument.c around dr_read_saved_reg at the reported assertion, then inspect the related dr_write_saved_reg check. Use the supplied call stack and the internal restore-state path through drreg_event_restore_state and instrument_restore_state to understand the legitimate cross-thread case. Done means these internal paths no longer trigger an overly strong ownership assertion while preserving validation for invalid uses.

Written by the indexing model from the issue text.