go/store/nbs: journal.go: Fix a bug which could result in broken databases after a process crash.

[reltuk]

If Dolt crashed immediately after creating an empty journal file, and before it wrote the initial set-root record to it, then the next time Dolt ran it would treat the root of the database as 0000....

This could also happen for certain observed filesystem states after an operating system crash.

[coffeegoddd]

@reltuk DOLT

read_tests	from_latency	to_latency	percent_change
covering_index_scan	0.55	0.55	0.0
groupby_scan	9.91	9.91	0.0
index_join	1.86	1.86	0.0
index_join_scan	1.34	1.34	0.0
index_scan	20.37	20.37	0.0
oltp_point_select	0.26	0.26	0.0
oltp_read_only	5.18	5.18	0.0
select_random_points	0.52	0.51	-1.92
select_random_ranges	0.57	0.56	-1.75
table_scan	20.74	20.74	0.0
types_table_scan	45.79	45.79	0.0

write_tests	from_latency	to_latency	percent_change
oltp_delete_insert	6.21	6.21	0.0
oltp_insert	3.07	3.02	-1.63
oltp_read_write	10.84	10.84	0.0
oltp_update_index	3.13	3.13	0.0
oltp_update_non_index	2.97	2.97	0.0
oltp_write_only	5.88	5.77	-1.87
types_delete_insert	6.55	6.55	0.0

[coffeegoddd]

@reltuk DOLT

test_name	from_latency_p95	to_latency_p95	percent_change
tpcc-scale-factor-1	45.79	45.79	0.0

test_name	from_server_name	from_server_version	from_tps	to_server_name	to_server_version	to_tps	percent_change
tpcc-scale-factor-1	dolt	55dc2b9	52.6	dolt	bed329a	52.81	0.4

[coffeegoddd]

@reltuk DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
bed329a	ok	5937471

version	total_tests
bed329a	5937471

correctness_percentage
100.0