Fix a potential hang if system clock jumped backward

[1495286789:156922][33496:0x7fde4f576700], t, lsm-worker: ../src/lsm/lsm_tree.c, 707: WT_TIMECMP( last_chunk->create_ts, prev_chunk->create_ts) >= 0

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#  RUN PARAMETERS

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abort=0

alter=0

auto_throttle=1

backups=0

bitcnt=8

bloom=1

bloom_bit_count=23

bloom_hash_count=14

bloom_oldest=0

cache=30

checkpoints=1

checksum=uncompressed

chunk_size=1

compaction=0

compression=none

data_extend=0

data_source=lsm

delete_pct=1

dictionary=0

direct_io=0

encryption=rotn-7

evict_max=2

file_type=row-store

firstfit=0

huffman_key=0

huffman_value=0

in_memory=0

insert_pct=17

internal_key_truncation=1

internal_page_max=17

isolation=read-committed

key_gap=7

key_max=119

key_min=32

leaf_page_max=16

leak_memory=0

logging=1

logging_archive=0

logging_compression=zlib

logging_prealloc=1

long_running_txn=0

lsm_worker_threads=4

merge_max=4

mmap=1

ops=100000

prefix_compression=1

prefix_compression_min=6

quiet=1

read_pct=0

rebalance=1

repeat_data_pct=26

reverse=1

rows=100000

runs=100

salvage=1

split_pct=81

statistics=0

statistics_server=0

threads=30

timer=20

transaction-frequency=6

value_max=1699

value_min=11

verify=1

wiredtiger_config=

write_pct=82

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