MINOR: thread: make "ti" a const pointer and clean up thread_info a bit

MINOR: thread: make "ti" a const pointer and clean up thread_info a bit

We want to make sure that the current thread_info accessed via "ti" will
remain constant, so that we don't accidentally place new variable parts
there and so that the compiler knows that info retrieved from there is
not expected to have changed between two function calls.

Only a few init locations had to be adjusted to use the array and the
rest is unaffected.