Support lift optimization for dag.Fork

[mattnibs]

Also fixes bug where the lift optimization would not get applied on eligible sequences with a length of 2.

Closes #4559, closes #4503, closes #5070

[nwt]

If this closes #4559, then shouldn't it lift a into the fork here? Never mind me. It does lift a into the fork.

$ zc -C -O 'fork ( => pass => pass ) | where a'
reader
| fork (
  =>
    pass
  =>
    pass
)
| where a

[philrz]

Aww yeah! I saw the #5070 reference so I ran its repro steps on the branch to see it's really true. 11+ minutes down to <18 seconds. Nice work @mattnibs!

$ zed -version
Version: v1.16.0-13-gf45df62a

$ time zed query 'from * | CuYcyl29a2g8pYV1s'
{_path:"files",ts:2017-03-24T19:59:23.399396Z,fuid:"F4NAir1FuTH6Oax2J4",tx_hosts:|[184.72.208.63]|,rx_hosts:|[10.10.7.65]|,conn_uids:|["CuYcyl29a2g8pYV1s"]|,source:"HTTP",depth:0(uint64),analyzers:|["MD5","SHA1","SHA256"]|,mime_type:"text/json",filename:null(string),duration:0s,local_orig:null(bool),is_orig:false,seen_bytes:35(uint64),total_bytes:null(uint64),missing_bytes:0(uint64),overflow_bytes:0(uint64),timedout:false,parent_fuid:null(string),md5:"dd82bbeb20a441bf55e62d42255de7f4",sha1:"90b01d6f0c6891651e8a729892e92592dd581762",sha256:"da1791b1b8361af6383790c6e77ef6eb149b22ed7e2d59b61ac2deb7884edf4d",extracted:null(string),extracted_cutoff:null(bool),extracted_size:null(uint64)}
{_path:"http",ts:2017-03-24T19:59:23.306076Z,uid:"CuYcyl29a2g8pYV1s",id:{orig_h:10.10.7.65,orig_p:64135(port=uint16),resp_h:184.72.208.63,resp_p:80(port)},trans_depth:1(uint64),method:"GET",host:"lc68.dsr.livefyre.com",uri:"/livecountping/98236508/663740012478?__=bye2msvgpn91&routed=1&jid&siteId=366439&networkId=livefyre.com",referrer:"http://www.asigra.com/blog/what-multi-tenancy-how-secure-it",version:"1.1",user_agent:"Mozilla/5.0 (Windows NT 10.0; WOW64; rv:52.0) Gecko/20100101 Firefox/52.0",request_body_len:0(uint64),response_body_len:35(uint64),status_code:200(uint64),status_msg:"OK",info_code:null(uint64),info_msg:null(string),tags:|[]|(|[zenum=string]|),username:null(string),password:null(string),proxied:null(|[string]|),orig_fuids:null([string]),orig_filenames:null([string]),orig_mime_types:null([string]),resp_fuids:["F4NAir1FuTH6Oax2J4"],resp_filenames:null([string]),resp_mime_types:["text/json"]}
{_path:"conn",ts:2017-03-24T19:59:23.231562Z,uid:"CuYcyl29a2g8pYV1s",id:{orig_h:10.10.7.65,orig_p:64135(port=uint16),resp_h:184.72.208.63,resp_p:80(port)},proto:"tcp"(=zenum),service:"http",duration:182.422ms,orig_bytes:438(uint64),resp_bytes:588(uint64),conn_state:"SF",local_orig:null(bool),local_resp:null(bool),missed_bytes:0(uint64),history:"ShADadfF",orig_pkts:5(uint64),orig_ip_bytes:650(uint64),resp_pkts:5(uint64),resp_ip_bytes:800(uint64),tunnel_parents:null(|[string]|)}

real	0m17.778s
user	0m0.033s
sys	0m0.037s