dialect/sql/sqlgraph: avoid creating tx blocks for single statement create operation

[a8m]

For example, in PostgreSQL, every statement is executed within a transaction. Therefore, we can avoid creating transaction
blocks manually (group of statements surrounded by BEGIN and COMMIT) for CreateNode operations with a single SQL statement.

Benchmark (based on @mattn reference) was improved from:

(2000 Inserts)   8.41s      4206748 ns/op    4595 B/op    115 allocs/op

To:

(2000 Inserts)   4.66s      2330222 ns/op    4107 B/op    104 allocs/op

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MySQL doc:

By default, MySQL runs with autocommit mode enabled. This means that, when not otherwise inside a transaction, each statement is atomic, as if it were surrounded by START TRANSACTION and COMMIT. You cannot use ROLLBACK to undo the effect; however, if an error occurs during statement execution, the statement is rolled back.

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SQLite doc:

Any command that accesses the database (basically, any SQL command, except a few PRAGMA statements) will automatically start a transaction if one is not already in effect. Automatically started transactions are committed when the last SQL statement finishes.