Document percona-202104 and percona-202105 new functionality

https://github.com/percona/percona-server/pull/4496

Support of `SELECT FOR UPDATE SKIP LOCKED/NOWAIT`:

It requires at least the transaction isolation level `READ COMMITTED`.

Cancelling ongoing manual compactions:

This version introduces two features to cancel ongoing manual compactions:

1. Client sessions running manual compactions by `SET GLOBAL rocksdb_compact_cf..` can be cancelled, like by Control+C (from the session) or KILL (from other sessions).
2. Introduced a global variable `rocksdb_cancel_manual_compactions` to cancel all ongoing manual compactions.

New variables:

1. Add a new variable "rocksdb_partial_index_sort_max_mem":

   :dyn: Yes
   :scope: Local
   :vartype: Unsigned Integer
   :default: 0
Maximum memory to use when sorting an unmaterialized group for partial indexes. 0 means no limit.

2. Add a new variable "rocksdb_bulk_load_partial_index":

   :dyn: Yes
   :scope: Local
   :vartype: Bool
   :default: ON
    Materialize partial index during bulk load, instead of leaving it empty.

3. Add a new variable "rocksdb_cancel_manual_compactions":

   :dyn: Yes
   :scope: Global
   :vartype: Bool
   :default: OFF
   Cancelling all ongoing manual compactions.