Add option to account history plugin to keep operations that happened in last X blocks in memory

PR for #2268.

Note: this triggers auto-replay.

Add 2 node startup options (available when the account_history plugin is enabled):

min-blocks-to-keep - Operations which are in the latest X blocks will be kept in memory. This option only takes effect when track-account is not used and max-ops-per-account is not zero. Note that this option may cause more history records to be kept in memory than the limit defined by the max-ops-per-account option, but the amount will be limited by the max-ops-per-acc-by-min-blocks option. The default value is 30,000 blocks.
max-ops-per-acc-by-min-blocks - A potential higher limit on the maximum number of operations per account to be kept in memory when the min-blocks-to-keep option causes the amount to exceed the limit defined by the max-ops-per-account option. If this is less than max-ops-per-account, max-ops-per-account will be used. The default value is 1,000 records.

bitshares / bitshares-core