Add two new sharder config options for configuring shrinking
behaviour:
- shrink_threshold: the size below which a shard may shrink
- expansion_limit: the maximum size to which an acceptor shard
may grow
The new options match the 'swift-manage-shard-ranges' command line
options and take absolute values.
The new options provide alternatives to the current equivalent options
'shard_shrink_point' and 'shard_shrink_merge_point', which are
expressed as percentages of 'shard_container_threshold'.
'shard_shrink_point' and 'shard_shrink_merge_point' are deprecated and
will be overridden by the new options if the new options are
explicitly set in a config file.
The default values of the new options are the same as the values that
would result from the default 'shard_container_threshold',
'shard_shrink_point' and 'shard_shrink_merge_point' i.e.:
- shrink_threshold: 100000
- expansion_limit: 750000
Change-Id: I087eac961c1eab53540fe56be4881e01ded1f60e
18f20daf38