Make OrcFile's "enforceBufferSize" user-settable.

Compression buffer-sizes in OrcFile are computed at runtime, except when enforceBufferSize is set. The only snag here is that this flag can't be set by the user.
When runtime-computed buffer-sizes are not optimal (for some reason), the user has no way to work around it by setting a custom value.

I have a patch that we use at Yahoo.