preserve data after database instance restart

[alex-dukhno]

Closes #38

Description

database use on disk persistence if it is run with PERSISTENT=1 env variable

Client output

user can work with previous dataset if start database with the following command PERSISTENT=1 RUST_LOG=debug cargo run

$ psql -h 127.0.0.1 -W
Password:
psql (12.3, server 0.0.0)
Type "help" for help.

alex-dukhno=> create schema schema_name;
CREATE SCHEMA
alex-dukhno=> create table schema_name.table_name (col1 integer);
CREATE TABLE
alex-dukhno=> insert into schema_name.table_name values (1);
INSERT 0 1
alex-dukhno=> select * from schema_name.table_name;
 col1
------
    1
(1 row)

alex-dukhno=> \q

after quitting psql and reconnecting to database

psql -h 127.0.0.1 -W
Password:
psql (12.3, server 0.0.0)
Type "help" for help.

alex-dukhno=> select * from schema_name.table_name;
 col1
------
    1
(1 row)

[coveralls]

Pull Request Test Coverage Report for Build 221036323

• 28 of 130 (21.54%) changed or added relevant lines in 5 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-1.5%) to 82.93%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/node/src/node.rs	0	1	0.0%
src/storage/src/persistent.rs	17	34	50.0%
src/sql_engine/src/catalog_manager/mod.rs	0	21	0.0%
src/sql_engine/src/catalog_manager/data_definition.rs	8	71	11.27%

Files with Coverage Reduction	New Missed Lines	%
src/storage/src/persistent.rs	1	74.12%

Totals
Change from base Build 220964973:	-1.5%
Covered Lines:	4416
Relevant Lines:	5325

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💛 - Coveralls