Integrate VShard into the core

[Gerold103]

Storage, track 1, replicaset- and cluster-wide spaces:

• Rename 'replicaset' to 'cluster' in some internal function, variable and constant names #3981 - Rename 'replicaset' to 'cluster' in some internal function, variable and constant names;
• New cluster-wide replication group #3982 - New cluster-wide replication group;
• Allow huge cluster size #3983 - Allow huge cluster size, blocked by New cluster-wide replication group #3982;
• Get rid of cluster full mesh #3984 - Get rid of cluster full mesh, blocked by Allow huge cluster size #3983.

Storage, track 2, native support of bucket id:

• Introduce system tuple fields #3985 - Introduce system tuple fields;
• Introduce _bucket system space #3986 - Introduce _bucket system space;
• Allow to specify/create a partitioning index on an ordinary space #3987 - Allow to specify/create a partitioning index on an ordinary space, blocked by Introduce _bucket system space #3986, Introduce system tuple fields #3985;
• Implement bucket_send function #3988 - Implement bucket_send function, blocked by Allow to specify/create a partitioning index on an ordinary space #3987.

Storage, track 3, sharding algorithms:

• Implement bucket garbage collector #3989 - Implement bucket garbage collector, blocked by Introduce _bucket system space #3986;
• Implement in-memory wal-free bucket references #3990 - Implement in-memory wal-free bucket references, blocked by Introduce _bucket system space #3986;
• Implement bucket recovery fiber #3991 - Implement bucket recovery fiber, blocked by Introduce _bucket system space #3986;
• Implement bucket rebalancer #3992 - Implement bucket rebalancer, blocked by Implement bucket_send function #3988.

Router:

• Allow proxy to connect to multiple replicasets #3993 - Allow proxy to connect to multiple replicasets, blocked by Implement a proxy module with manual failover #2625;
• Allow to specify bucket_id with any request on proxy #3994 - Allow to specify bucket_id with any request on proxy, blocked by Introduce _bucket system space #3986, Allow proxy to connect to multiple replicasets #3993;
• Implement bucket routing table, bucket discovery, point lookup of bucket #3995 - Implement bucket routing table, bucket discovery, point lookup of bucket, blocked by Allow to specify bucket_id with any request on proxy #3994.