We have a fair amount of complexity (EqualSplit etc) involved in dynamically
splitting files down into chunks based on available concurrency.

We could be less clever and use a fixed chunk size, with a fixed buffer size to
go with it.  This would dovetail nicely with a sync.Pool of fixed-size buffers,
and also with a configuration option to limit the total allocated
RAM (implemented as a semaphore guarding how many buffers can be fetched from
the sync.Pool).

This commit only does consistent hashing mode; we should do buffer mode as well
if we do this work.

For const and var shared between buffer and consistent-hash mode move these
constants to a dedicated .go file. These are two distinct downloaders
and neither should be responsible for explicitly shared elements.

Wire up the --chunk-size option and deprecate --minimum-chunk-size.

The concurrency calculation is no longer used in consistent-hashing.
Remove it.

Update Buffer downloader to use ChunkSize instead of MinChunkSize.
Options have been modified to only contain ChunkSize now as the root and
multifile commands no longer reference MinChunkSize.

This change is in support of moving towards simplifying the chunksize
logic. Additionally this will make reuse of buffers significantly easier
as the next phase.

If chunkSize doesn't divide contentSize evenly, and chunkSize < 0.5*contentSize,
we try to download a chunk bigger than chunkSize.  This should not be possible.

Every chunk except the last is now the same chunkSize, which is the intention of
the code.  We no longer dynamically change chunk size.

Also, simplify the consistent hashing code to be more similar to buffer mode,
and remove the dead EqualSplit code.

Aliases in Cobra and Viper are relatively complex and
viper.RegisterAlias does not do what would be expected as ENV handling
is a very special case. This commit fixes up the handling of aliases
specifically for chunk size and minimuim chunk size.

* CLI Option Set
* NEW ENV VAR
* Deprecated ENV Var
* Default Value

For ChunkSize and MinimumChunkSize we now explicitly disallow setting
both --chunk-size and --minimum-chunk-size as CLI options.

If PGET_CHUNK_SIZE is set, we utilize that value except if the cli
option(s) are set.

If PGET_MINIMUM_CHUNK_SIZE is set
* If PGET_CHUNK_SIZE is set to a non-default value a warning is emitted,
  and PGET_CHUNK_SIZE is used..

* If PGET_CHUNK_SIZE is the default value, PGET_MINIMUM_CHUNK_SIZE is
  set as PGET_CHUNK_SIZE and a deprecation warning is emitted.

Additionally, viper calls now all reference config.OptChunkSize

Simplify conditional.

Also, if both PGET_MINIMUM_CHUNK_SIZE and PGET_CHUNK_SIZE are set to the same
value, don't emit a warning.