Rework the DBDelegate implementation to use ObjC properties

[MrRooni]

We're getting crashes in 1Password with the following backtrace hits:

Incident Identifier: 8BE586D6-F587-4A20-B96A-9C1B5100534C
CrashReporter Key:   95FB86FB-D31E-43AE-9996-2362BEDADC83
Hardware Model:      iPhone7,1
Process:         1Password [1511]
Path:            /var/containers/Bundle/Application/53A3C1E7-672E-4742-9A7D-DB672FBB4998/1Password.app/1Password
Identifier:      com.agilebits.onepassword-ios
Version:         7.2.1 (70201002)
Code Type:       ARM-64
Parent Process:  ??? [1]

Date/Time:       2018-10-11T02:44:50Z
Launch Time:     2018-10-11T02:44:42Z
OS Version:      iPhone OS 12.0.1 (16A404)
Report Version:  104

Exception Type:  SIGSEGV
Exception Codes: SEGV_ACCERR at 0x0
Crashed Thread:  6

Application Specific Information:
Selector name found in current argument registers: hash

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.
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Thread 6 Crashed:
0   CoreFoundation                       0x00000001bd1d7010 -[__NSDictionaryM setObject:forKeyedSubscript:] + 484
1   ObjectiveDropboxOfficial             0x0000000101790050 -[DBDelegate sessionDataWithSession:] (DBDelegate.m:334)
2   ObjectiveDropboxOfficial             0x000000010178eeb0 __81-[DBDelegate addRpcResponseHandler:session:responseHandler:responseHandlerQueue:]_block_invoke (DBDelegate.m:239)
3   Foundation                           0x00000001bdd73b6c __NSBLOCKOPERATION_IS_CALLING_OUT_TO_A_BLOCK__ + 12
4   Foundation                           0x00000001bdc7bcc8 -[NSBlockOperation main] + 68
5   Foundation                           0x00000001bdc7b19c -[__NSOperationInternal _start:] + 736
6   Foundation                           0x00000001bdd75a40 __NSOQSchedule_f + 268
7   libdispatch.dylib                    0x00000001bcd1f6c8 _dispatch_call_block_and_release + 20
8   libdispatch.dylib                    0x00000001bcd20484 _dispatch_client_callout + 12
9   libdispatch.dylib                    0x00000001bccc3874 _dispatch_continuation_pop$VARIANT$mp + 408
10  libdispatch.dylib                    0x00000001bccc2f3c _dispatch_async_redirect_invoke + 596
11  libdispatch.dylib                    0x00000001bcccfa60 _dispatch_root_queue_drain + 372
12  libdispatch.dylib                    0x00000001bccd0308 _dispatch_worker_thread2 + 124
13  libsystem_pthread.dylib              0x00000001bcf02190 _pthread_wqthread + 468
14  libsystem_pthread.dylib              0x00000001bcf04d00 start_wqthread + 0

This PR starts with some small cosmetic and safety changes.

[lobianco]

Redundant check?

[rudyrichter]

if it fails should it be allowed to wipe out what's there?

[chadseld]

shadowing the sessionData property makes this more confusing.

[rudyrichter]

I don't think we'll end up merging this one.