Instruction updates with insert and remove, mission revision and status details

[jp-pino]

SetInstructionUpdateReq is not used by any client yet, so it is redesigned to cover replace, insert and remove, with a real answer:

message SetInstructionUpdateReq {
  oneof operation {
    Instruction replace = 1;       // same field number and type as before
    InsertInstruction insert = 2;  // { instruction, before_id (0 appends) }
    uint32 remove_id = 3;
  }
}
message SetInstructionUpdateRep { bool accepted = 1; string reason = 2; uint32 revision = 3; }

Also:

• Mission.revision, increased by the drone with every accepted update.
• MissionStatus.active_instruction_id, revision, failure_reason.
• Instruction.auto_continue renamed to pause_on_completion (same number and type): the field was never set by clients, so with proto3's default the old meaning would have paused after every instruction. True now pauses after the instruction; old missions keep running throughout.
• NOTIFICATION_TYPE_MISSION_PAUSED_BY_INSTRUCTION (36) and NOTIFICATION_TYPE_MISSION_UPDATED (37).

Depends on #303.