crimson/os/seastore/EPM: refactor to index writers by hint

crimson/os/seastore/EPM: refactor to index writers by hint

Generally, simplify the indexing from:
EPM ==(device-id)=> Allocator ==> Writers
to:
EPM ==(hint)=> Writers

This is for the following considerations:
* Writer may not be specific to a device;
* Faster placement decision making directly by hint;

Signed-off-by: Yingxin Cheng <yingxin.cheng@intel.com>