scylla-gdb: add io-queues command
scylla-gdb: add io-queues command

The command can be used to inspect reactor's IO queues. Example output:
(gdb) scylla io-queues
    Dev 0:
	Class:                  |shares:         |ptr:
	--------------------------------------------------------------------------------
        "default"               |1               |(seastar::priority_class_data *)0x6000002c6500
        "commitlog"             |1000            |(seastar::priority_class_data *)0x6000003ad940
        "memtable_flush"        |1000            |(seastar::priority_class_data *)0x6000005cb300
        "streaming"             |200             |(seastar::priority_class_data *)0x0
        "query"                 |1000            |(seastar::priority_class_data *)0x600000718580
        "compaction"            |1000            |(seastar::priority_class_data *)0x6000030ef0c0

        Max request size:    2147483647
        Max capacity:        Ticket(weight: 4194303, size: 4194303)
        Capacity tail:       Ticket(weight: 73168384, size: 100561888)
        Capacity head:       Ticket(weight: 77360511, size: 104242143)

        Resources executing: Ticket(weight: 2176, size: 514048)
        Resources queued:    Ticket(weight: 384, size: 98304)
        Handles: (1)
            Class 0x6000005d7278:
                Ticket(weight: 128, size: 32768)
                Ticket(weight: 128, size: 32768)
                Ticket(weight: 128, size: 32768)
        Pending in sink: (0)

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla