Add option to enforce fetching data from local shards, instead of from shards on on remote nodes(if data present on local node)

Add option to enforce fetching data from local shards, instead of from shards/copies on on the remote nodes(if data present on local node), something like GET **/mydb/doc1?preferlocal=true**.

## Summary

In the current software, it seems CouchDB will always try to distribute disk i/o across nodes during query.

For example: in a two nodes cluster(MACHINE1.SOMECOMPANY.COM and MACHINE2.SOMECOMPANY.COM), database mydb was created with q=8 and n=2, as following. Each node contains a copy of all the data. **Documents in the example database contain large binary data(i.e, as attachments.**)
````
GET /_membership
{
"all_nodes": [
"couchdb@MACHINE1.SOMECOMPANY.COM",
"couchdb@MACHINE2.SOMECOMPANY.COM"
],
"cluster_nodes": [
"couchdb@MACHINE1.SOMECOMPANY.COM",
"couchdb@MACHINE2.SOMECOMPANY.COM"
]
}

GET /mydb
{
"db_name": "mydb",
"update_seq": "167626-g1AAAAGbeJzLYWBg4MhgTmEQT84vTc5ISXIIcAn2MTLRcw0O8tRz9vfNASpgSmRIkv___39WEgND4B2CqpMUgGSSPUzDbEwNZmgaHEAa4mEazAhrSABpqIdqCHImqCGPBUgyNAApoJ75YE0TiNS0AKJpP9hpxGo6ANF0n8gAgGh6ANEE8ZN8FgBSO3e3",
"sizes": {
"file": 31189227296,
"external": 30639090294,
"active": 30491445284
},
"purge_seq": 0,
"other": {
"data_size": 30639090294
},
"doc_del_count": 0,
"doc_count": 166294,
"disk_size": 31189227296,
"disk_format_version": 6,
"data_size": 30491445284,
"compact_running": false,
"cluster": {
"q": 8,
"n": 2,
"w": 2,
"r": 2
},
"instance_start_time": "0"
}

GET /mydb/_shards
{
"shards": {
"00000000-1fffffff": [
"couchdb@MACHINE1.SOMECOMPANY.COM",
"couchdb@MACHINE2.SOMECOMPANY.COM"
],
"20000000-3fffffff": [
"couchdb@MACHINE1.SOMECOMPANY.COM",
"couchdb@MACHINE2.SOMECOMPANY.COM"
],
"40000000-5fffffff": [
"couchdb@MACHINE1.SOMECOMPANY.COM",
"couchdb@MACHINE2.SOMECOMPANY.COM"
],
"60000000-7fffffff": [
"couchdb@MACHINE1.SOMECOMPANY.COM",
"couchdb@MACHINE2.SOMECOMPANY.COM"
],
"80000000-9fffffff": [
"couchdb@MACHINE1.SOMECOMPANY.COM",
"couchdb@MACHINE2.SOMECOMPANY.COM"
],
"a0000000-bfffffff": [
"couchdb@MACHINE1.SOMECOMPANY.COM",
"couchdb@MACHINE2.SOMECOMPANY.COM"
],
"c0000000-dfffffff": [
"couchdb@MACHINE1.SOMECOMPANY.COM",
"couchdb@MACHINE2.SOMECOMPANY.COM"
],
"e0000000-ffffffff": [
"couchdb@MACHINE1.SOMECOMPANY.COM",
"couchdb@MACHINE2.SOMECOMPANY.COM"
]
}
}
````
When querying/fetching data from mydb, it seems CouchDB may load some data from local shards, and some from shards on the remote machine. Due to the large document size( thus the heavy network traffic to to transfer data from the remote note the the coordinate node), the query speed is much slower than the standalone deployment.

**Query is 2+ times faster with the standalone deployment for the same testing data.**

Adding ?r=1 did not help.

## Possible Solution

It will be very helpful to add an option in the query request, something like **GET /mydb/id0?preferlocal=true**, to enforce fetching data from shards on local node if it presents locally; otherwise, fetching from remote shards.

## Additional context

Software Tested: CouchDB 2.2
OS: Linux.

Start with the GET /mydb/{document-id} request path and the shard routing represented by GET /mydb/_shards. Determine how a request chooses local versus remote shard copies and how a preferlocal=true option should behave when no local copy is available. Done means the option is documented, covered by tests for local and fallback routing, and does not change default behavior.

Written by the indexing model from the issue text.