Update reverse modify logic to always insert a modify into the history store

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ERROR: test_las01.test_las01.test_las (subunit.RemotedTestCase)

test_las01.test_las01.test_las

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_StringException: Traceback (most recent call last):

  File "/mnt/data0/chenhaoqu/work/wiredtiger/test/suite/test_las01.py", line 155, in test_las

    self.durable_check(bigvalue4, uri, ds, nrows)

  File "/mnt/data0/chenhaoqu/work/wiredtiger/test/suite/test_las01.py", line 76, in durable_check

    self.session.checkpoint()

WiredTigerError: No such file or directory

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[1579243684:937132][4052:0x7fe4d5597740], test_las01.test_las01.test_las, WT_CONNECTION.close: __conn_close, 1096: failure during close, disabling further writes: WT_NOTFOUND: item not found

[1579243685:453437][4052:0x7fe4d5597740], test_las01.test_las01.test_las, WT_CONNECTION.close: __wt_cache_destroy, 348: cache server: exiting with 2992 pages in memory and 1708 pages evicted

[1579243685:453632][4052:0x7fe4d5597740], test_las01.test_las01.test_las, WT_CONNECTION.close: __wt_cache_destroy, 353: cache server: exiting with 10259385 image bytes in memory

[1579243685:453662][4052:0x7fe4d5597740], test_las01.test_las01.test_las, WT_CONNECTION.close: __wt_cache_destroy, 356: cache server: exiting with 21128876 bytes in memory

[1579243685:453675][4052:0x7fe4d5597740], test_las01.test_las01.test_las, WT_CONNECTION.close: __wt_cache_destroy, 362: cache server: exiting with 18324966 bytes dirty and 1249 pages dirty