[DocDB] Tablet splitting: single tablet first split may produce an empty child tablet

[arybochkin]

Jira Link: DB-4299

Description

Depending on keys structure in rocksdb, there can be a situation where a tablet contains the same hash code for the majority of key. This leads to a situation where the middle split key matches the very first key and such key selection leads to appearing an empty child. The issue is applicable only for hash partitioned tables and only for the first split of a single tablet (table has only one tablet).
Example:

create table demo (h int, r int generated always as identity, data text, primary key(h hash, r asc) );
copy demo(h, data) from program 'base64 -w 16384 /dev/urandom | head -c 838860800 | awk ''{ print 1, $0 }''' delimiter ' ';

This generates 800MB data of 51197 rows with h=1, r=1..51197, data - random 16KB text and results in the following split:
hash_split: [0x0000, 0x120F] (depth 1) - 0 bytes of SST files
hash_split: [0x1210, 0x5554] (depth 1) - 800MB of SST files

@ttyusupov