legate/core: fix FutureMap leak in communicator shutdown

[rohany]

This commit fixes the following leak, leading to shutdown hangs.

found cycle!
  tail:
    0x200092ca1e10: <class 'legate.core._legion.future.FutureMap'>
     ^
    0x200058dd65c0: <class 'dict'>
     ^ ["_handles"]
    0x2000588dff80: <class 'dict'>
     ^ .__dict__
    0x2000889b11b0: <class 'legate.core.communicator.NCCLCommunicator'>
     ^ ["_nccl"]
    0x2000588dd540: <class 'dict'>
     ^ .__dict__
    0x2000887b8430: <class 'legate.core.runtime.CommunicatorManager'>
     ^ ["_comm_manager"]
    0x2000889f7d00: <class 'dict'>
     ^ .__dict__
    0x2000887b9f90: <class 'legate.core.runtime.Runtime'>
     ^ ["runtime"]
    0x200173d7b180: <class 'dict'>
  cycle:
    0x200173d7b180: <class 'dict'>
     ^ .__dict__
    0x200173d84370: <class 'legate.core.runtime.ConsensusMatchingFieldManager'>
     ^ ["manager"]
    0x200173e30cc0: <class 'dict'>
     ^ .__dict__
    0x200173db2e00: <class 'legate.core.runtime.FreeFieldInfo'>
     ^ [85]
    0x200173df0f80: <class 'list'>
     ^ ["_freed_fields"]
    0x2000588ddb40: <class 'dict'>
     ^ .__dict__
    0x2000889b0fa0: <class 'legate.core.runtime.FieldMatchManager'>
     ^ ["_field_match_manager"]
    0x200173d7b180: <class 'dict'>

Signed-off-by: Rohan Yadav rohany@alumni.cmu.edu