Lock access to control->symtab to prevent data race

WARNING: ThreadSanitizer: data race
    Read of size 8 at 0x000000000001 by thread T1:
    #0 isccc_symtab_foreach lib/isccc/symtab.c:277:14
    #1 isccc_cc_cleansymtab lib/isccc/cc.c:954:2
    #2 control_recvmessage bin/named/controlconf.c:477:2
    #3 recv_data lib/isccc/ccmsg.c:110:2
    #4 read_cb lib/isc/netmgr/tcp.c:769:4
    #5 <null> <null>

    Previous write of size 8 at 0x000000000001 by thread T2:
    #0 isccc_symtab_define lib/isccc/symtab.c:242:2
    #1 isccc_cc_checkdup lib/isccc/cc.c:1026:11
    #2 control_recvmessage bin/named/controlconf.c:478:11
    #3 recv_data lib/isccc/ccmsg.c:110:2
    #4 read_cb lib/isc/netmgr/tcp.c:769:4
    #5 <null> <null>

    Location is heap block of size 190352 at 0x000000000011 allocated by main thread:
    #0 malloc <null>
    #1 isccc_symtab_create lib/isccc/symtab.c:76:18
    #2 isccc_cc_createsymtab lib/isccc/cc.c:948:10
    #3 named_controls_create bin/named/controlconf.c:1483:11
    #4 named_server_create bin/named/server.c:10057:2
    #5 setup bin/named/main.c:1256:2
    #6 main bin/named/main.c:1523:2

    Thread T1 (running) created by main thread at:
    #0 pthread_create <null>
    #1 isc_thread_create lib/isc/pthreads/thread.c:73:8
    #2 isc_nm_start lib/isc/netmgr/netmgr.c:215:3
    #3 create_managers bin/named/main.c:909:15
    #4 setup bin/named/main.c:1223:11
    #5 main bin/named/main.c:1523:2

    Thread T2 (running) created by main thread at:
    #0 pthread_create <null>
    #1 isc_thread_create lib/isc/pthreads/thread.c:73:8
    #2 isc_nm_start lib/isc/netmgr/netmgr.c:215:3
    #3 create_managers bin/named/main.c:909:15
    #4 setup bin/named/main.c:1223:11
    #5 main bin/named/main.c:1523:2

    SUMMARY: ThreadSanitizer: data race lib/isccc/symtab.c:277:14 in isccc_symtab_foreach

(cherry picked from commit 0450acc1)

Closes #2131 (closed)