ZKL: HistoryManager.cs: reduce ChatTab initialization time. 

(When channel is initialized, read only last 3kb of chat history to append to chat window, not open the whole file)
huge improvement in startup time if we have large chat history (at least 1 second is saved)

Test result:
hour : minute : second : millisecond/10 = 00:00:00.00
14/10/2014 1:05:01 PM: TASC channel joined: zkadmin
14/10/2014 1:05:01 PM: File.ReadA read 00:00:00.02
14/10/2014 1:05:01 PM: FileStream read 00:00:00.01
14/10/2014 1:05:01 PM: TASC channel joined: clan_ISP
14/10/2014 1:05:01 PM: File.ReadA read 00:00:00.00
14/10/2014 1:05:01 PM: FileStream read 00:00:00.00
14/10/2014 1:05:01 PM: TASC channel joined: Rising
14/10/2014 1:05:02 PM: File.ReadA read 00:00:00.01
14/10/2014 1:05:02 PM: FileStream read 00:00:00.00
14/10/2014 1:05:02 PM: TASC channel joined: zk
14/10/2014 1:05:02 PM: File.ReadA read 00:00:00.67
14/10/2014 1:05:02 PM: FileStream read 00:00:00.00
14/10/2014 1:05:03 PM: TASC channel joined: zkdev
14/10/2014 1:05:03 PM: File.ReadA read 00:00:00.44
14/10/2014 1:05:03 PM: FileStream read 00:00:00.00
14/10/2014 1:05:03 PM: TASC channel joined: zkcore
14/10/2014 1:05:03 PM: File.ReadA read 00:00:00.00
14/10/2014 1:05:03 PM: FileStream read 00:00:00.00
14/10/2014 1:05:03 PM: TASC channel joined: sy
14/10/2014 1:05:03 PM: File.ReadA read 00:00:00.09
14/10/2014 1:05:03 PM: FileStream read 00:00:00.00
14/10/2014 1:05:03 PM: TASC channel joined: en
14/10/2014 1:05:03 PM: File.ReadA read 00:00:00.00
14/10/2014 1:05:03 PM: FileStream read 00:00:00.00

tried again in reverse, just in case:
14/10/2014 1:15:25 PM: TASC channel joined: zkadmin
14/10/2014 1:15:25 PM: FileStream read 00:00:00.00
14/10/2014 1:15:25 PM: File.ReadA read 00:00:00.02
14/10/2014 1:15:25 PM: TASC channel joined: clan_ISP
14/10/2014 1:15:25 PM: FileStream read 00:00:00.00
14/10/2014 1:15:25 PM: File.ReadA read 00:00:00.00
14/10/2014 1:15:25 PM: TASC channel joined: Rising
14/10/2014 1:15:25 PM: FileStream read 00:00:00.00
14/10/2014 1:15:25 PM: File.ReadA read 00:00:00.00
14/10/2014 1:15:26 PM: TASC channel joined: zk
14/10/2014 1:15:26 PM: FileStream read 00:00:00.00
14/10/2014 1:15:26 PM: File.ReadA read 00:00:00.34
14/10/2014 1:15:26 PM: TASC channel joined: zkdev
14/10/2014 1:15:26 PM: FileStream read 00:00:00.00
14/10/2014 1:15:27 PM: File.ReadA read 00:00:00.45
14/10/2014 1:15:27 PM: TASC channel joined: zkcore
14/10/2014 1:15:27 PM: FileStream read 00:00:00.00
14/10/2014 1:15:27 PM: File.ReadA read 00:00:00.00
14/10/2014 1:15:27 PM: TASC channel joined: sy
14/10/2014 1:15:27 PM: FileStream read 00:00:00.00
14/10/2014 1:15:27 PM: File.ReadA read 00:00:00.17
14/10/2014 1:15:27 PM: TASC channel joined: en
14/10/2014 1:15:27 PM: FileStream read 00:00:00.00
14/10/2014 1:15:27 PM: File.ReadA read 00:00:00.00