GetMemory/GetSpan takes a minimum size (dotnet/corefx#35186)

GetMemory/GetSpan takes a minimum size (dotnet/corefx#35186)

- We changed the contract of IBufferWriter to be more usable by making the sizeHint a minimum size. If the caller requests memory over the max pool size then we allocate.
- In the default case where we're using the shared array pool, we let the underlying pool handle this (it does internally).

Commit migrated from https://github.com/dotnet/corefx/commit/c45d88d5a1326501e31c02e508efb352a492fb59