feat(inserter): start new insert only when the first row is provided

[laplab]

Benchmarks

No noticeable regression was detected.

$ critcmp master patch
group                    master                                 patch
-----                    ------                                 -----
insert/lz4               1.00    84.9±66.97ns   269.6 MB/sec    1.04    88.0±67.45ns   260.2 MB/sec
insert/lz4hc(4)          1.00    89.0±69.89ns   257.1 MB/sec    1.02    90.6±65.06ns   252.8 MB/sec
insert/no compression    1.00     38.0±5.62ns   603.0 MB/sec    1.03     39.1±7.13ns   585.3 MB/sec

[loyd]

insert/no compression   time:   [30.730 ns 31.099 ns 31.432 ns]
                        thrpt:  [728.18 MiB/s 735.98 MiB/s 744.81 MiB/s]
                 change:
                        time:   [-1.5399% +0.0178% +1.6136%] (p = 0.98 > 0.05)
                        thrpt:  [-1.5879% -0.0178% +1.5639%]
                        No change in performance detected.
insert/lz4              time:   [44.406 ns 46.780 ns 49.325 ns]
                        thrpt:  [464.03 MiB/s 489.27 MiB/s 515.43 MiB/s]
                 change:
                        time:   [-19.679% +0.4489% +25.816%] (p = 0.96 > 0.05)
                        thrpt:  [-20.519% -0.4469% +24.500%]
                        No change in performance detected.
insert/lz4hc(4)         time:   [48.988 ns 50.922 ns 53.148 ns]
                        thrpt:  [430.65 MiB/s 449.48 MiB/s 467.22 MiB/s]
                 change:
                        time:   [-17.625% +2.7077% +29.089%] (p = 0.82 > 0.05)
                        thrpt:  [-22.534% -2.6363% +21.396%]
                        No change in performance detected.

ok