query for unresolved instances only when needed

[keepsimple1]

This patch is to solve issue #156 . The main change is to add a new internal command Command::Resolve to query unresolved instances. Such commands are only issued for active queries and are limited to twice, per 0.5 second.

Ran manual test:

• change broadcast_service_on_intf to only send PTR record, not SRV, TXT records.
• added println to show the Command::Resolve triggered properly.
• verified SRV query only sent once and stopped when successful.
• verified pending_resolves updated properly.

manual test log:

At 3.571128911s : ServiceFound("_no-srv._tcp.local.", "test3._no-srv._tcp.local.")
SystemTime { tv_sec: 1703194637, tv_nsec: 434398000 } exec Command::Resolve for test3._no-srv._tcp.local. try_count 1
SystemTime { tv_sec: 1703194637, tv_nsec: 434462000 } send query for TYPE_ANY for test3._no-srv._tcp.local.
At 4.072879335s: Resolved a new service: test3._no-srv._tcp.local. host: mdns-example.local. port: 3456 IP: {fe80::25ca:74bd:9126:8e57, fe80::1, fe80::235d:96fc:aee0:2a0c, fe80::1846:93a2:ca2a:a09b, fe80::ce81:b1c:bd2c:69e, fe80::8511:a836:fa4a:726f, fe80::70ca:4bb:f4a5:e9a8, 192.168.0.107, fe80::e5a8:1e17:be1:b792, fe80::dc77:60ff:fe7b:ad42, fe80::b0e8:4591:c003:25b0} TXT properties: TxtProperties { properties: [TxtProperty {key: "PATH", val: Some("one")}] }
SystemTime { tv_sec: 1703194637, tv_nsec: 934134000 } exec Command::Resolve for test3._no-srv._tcp.local. try_count 2
SystemTime { tv_sec: 1703194637, tv_nsec: 934159000 } SRV record already exists for test3._no-srv._tcp.local.

test log for pending_resolves:

At 3.76635187s : ServiceFound("_no-srv._tcp.local.", "test3._no-srv._tcp.local.")
adding a new pending resolve
pending_resolves count: 1
pending_resolves count: 1
pending_resolves count: 1
pending_resolves count: 1
exec Command::Resolve for test3._no-srv._tcp.local. try_count 1
send query for missing SRV test3._no-srv._tcp.local.
pending_resolves count: 0
At 4.269338215s: Resolved a new service: test3._no-srv._tcp.local. host: mdns-example.local. port: 3456 IP: {fe80::25ca:74bd:9126:8e57, fe80::e5a8:1e17:be1:b792, fe80::1846:93a2:ca2a:a09b, fe80::235d:96fc:aee0:2a0c, fe80::ce81:b1c:bd2c:69e, fe80::1, fe80::b0e8:4591:c003:25b0, 192.168.0.107, fe80::70ca:4bb:f4a5:e9a8, fe80::8511:a836:fa4a:726f, fe80::dc77:60ff:fe7b:ad42} TXT properties: TxtProperties { properties: [TxtProperty {key: "PATH", val: Some("one")}] }
pending_resolves count: 0
pending_resolves count: 0
exec Command::Resolve for test3._no-srv._tcp.local. try_count 2
pending_resolves count: 0