Bitshares-core: Market history plugin: `history-per-size` option doesn't work

[abitmore]

Old data never got removed.

[oxarbitrage]

reproduced, it seems to be a problem around here: https://github.com/bitshares/bitshares-core/blob/master/libraries/plugins/market_history/market_history_plugin.cpp#L129 and https://github.com/bitshares/bitshares-core/blob/master/libraries/plugins/market_history/market_history_plugin.cpp#L202

ill check it more in deep tomorrow and try to fix it.

[abitmore]

@oxarbitrage I'll handle this. Thank you anyway.

[abitmore]

Test result with current BitShares main net (head block #21742070) and following options:

# plugins =
# track-account =
partial-operations = 1
max-ops-per-account = 1000
bucket-size = [60,300,900,1800,3600,14400,86400]
history-per-size = 1000

running in Ubuntu 16.04,
before fix: VIRT = 6829564, RES = 5.272g
after fix: VIRT = 5982640, RES = 4.805g

So the fix saves 470M ~ 850M RAM.

[oxarbitrage]

fixed at https://github.com/bitshares/bitshares-core/pull/478