exponential capacity growth for io::TypedIOBuf

exponential capacity growth for io::TypedIOBuf

Summary:
Amortized cost of `TypedIOBuf::push()` is `O(N)` at this moment.
Unless we're using jemalloc where we can rely on `realloc()`.  In this
diff we switch to exponential growth otherwise.

Test Plan:
built unicorn/test with ASan (and thus without jemalloc), ran
end-to-end tests and verified we no longer spend minutes to build di4

@override-unit-failures

Reviewed By: tudorb@fb.com

FB internal diff: D1015233