data drop sending long strings

Hi,
according to several issues adressing data drop, my problem could not be solved by those fixes yet.

Setup:
Win10 x64
Arduino IDE 1.8.6
bleuart sketch in full debug mode
Android Bluefruit LE App

Sending string from Bluefruit LE App via Uart to the Board.

Example String to send:
```
{
"id": 1,
"name": "Leanne Graham",
"username": "Bret",
"email": "Sincere@april.biz",
"address": {
"street": "Kulas Light",
"suite": "Apt. 556",
"city": "Gwenborough",
"zipcode": "92998-3874",
"geo": {
"lat": "-37.3159",
"lng": "81.1496"
}
},
"phone": "1-770-736-8031 x56442",
"website": "hildegard.org",
"company": {
"name": "Romaguera-Crona",
"catchPhrase": "Multi-layered client-server neural-net",
"bs": "harness real-time e-markets"
}
}
```

What I reveived:
```
{ "id": 1, "name": "Leanne Graham", "use{ "lat": "-37.3159",na", "catchPhrase": t-server neural-net"l-time e-markets" } }
```

I tried the pull request #178 , but it could not solve this problem.

Is there any other boundry, why this might not work?

What I realised is that up to 20 bytes it's safe to send data. Bigger junks are very likely to drop data.

Full Serial Log:
```
Bluefruit52 BLEUART Example
---------------------------

[CFG ] SoftDevice's RAM requires: 0x20002CE8
Please use Adafruit's Bluefruit LE app to connect in UART mode
Once connected, enter character(s) that you wish to send

BSP Library : 0.8.5
Firmware : S132 5.1.0, 5.1.0 dual banks
Serial No : 6F3Fxxxxx

--------- SoftDevice Config ---------
Max UUID128 : 10
ATTR Table Size : 2048
Service Changed : 0
Peripheral Connect Setting
- Max MTU : 247
- Event Length : 6
- HVN Queue Size : 3
- WrCmd Queue Size: 1

--------- BLE Settings ---------
Name : Bluefruit52
Max Connections : Peripheral = 1, Central = 0
Address : FD:9D:0C:BA:D2:AF (Static)
TX Power : 4 dBm
Conn Intervals : min = 20.00 ms, max = 30.00 ms
Peripheral Paired Devices:

Central Paired Devices:

[BLE ] BLE_GAP_EVT_CONNECTED : Conn Handle = 0
[GAP ] Conn Interval= 48.750000
[BLE ] BLE_GATTC_EVT_CHAR_VAL_BY_UUID_READ_RSP : Conn Handle = 0
[GATTS ] Confirm received handle = 0x0000
Connected to Galaxy S7
[BLE ] BLE_GAP_EVT_CONN_PARAM_UPDATE : Conn Handle = 0
[GAP ] Conn Interval= 30.000000
[BLE ] BLE_GAP_EVT_CONN_PARAM_UPDATE : Conn Handle = 0
[GAP ] Conn Interval= 7.500000
[BLE ] BLE_GAP_EVT_CONN_PARAM_UPDATE : Conn Handle = 0
[GAP ] Conn Interval= 30.000000
[BLE ] BLE_GATTS_EVT_WRITE : Conn Handle = 0
[GATTS ] attr's cccd
00: 01 00
[BLE ] BLE_GATTS_EVT_WRITE : Conn Handle = 0
[GATTS ] attr's cccd
00: 00 00
[BLE ] BLE_GATTS_EVT_SYS_ATTR_MISSING : Conn Handle = 0
[BLE ] BLE_GATTS_EVT_WRITE : Conn Handle = 0
[GATTS ] attr's cccd
00: 01 00
[BLE ] BLE_GATTS_EVT_WRITE : Conn Handle = 0
[GATTS ] attr's value, uuid = 0x0002
00: 7B 20 22 69 64 22 3A 20 31 2C 20 22 6E 61 6D 65
10: 22 3A 20 22
[BLEUART] RX:
data: 7b 20 22 69 64 22 3a 20 31 2c 20 22 6e 61 6d 65 22 3a 20 22
[BLE ] BLE_GATTS_EVT_WRITE : Conn Handle = 0
[GATTS ] attr's value, uuid = 0x0002
00: 4C 65 61 6E 6E 65 20 47 72 61 68 61 6D 22 2C 20
10: 22 75 73 65
[BLEUART] RX:
data: 4c 65 61 6e 6e 65 20 47 72 61 68 61 6d 22 2c 20 22 75 73 65
[BLE ] BLE_GATTS_EVT_WRITE : Conn Handle = 0
[GATTS ] attr's value, uuid = 0x0002
00: 7B 20 22 6C 61 74 22 3A 20 22 2D 33 37 2E 33 31
10: 35 39 22 2C
[BLEUART] RX:
data: 7b 20 22 6c 61 74 22 3a 20 22 2d 33 37 2e 33 31 35 39 22 2c
[BLE ] BLE_GATTS_EVT_WRITE : Conn Handle = 0
[GATTS ] attr's value, uuid = 0x0002
00: 6E 61 22 2C 20 22 63 61 74 63 68 50 68 72 61 73
10: 65 22 3A 20
[BLEUART] RX:
data: 6e 61 22 2c 20 22 63 61 74 63 68 50 68 72 61 73 65 22 3a 20
[BLE ] BLE_GATTS_EVT_WRITE : Conn Handle = 0
[GATTS ] attr's value, uuid = 0x0002
00: 74 2D 73 65 72 76 65 72 20 6E 65 75 72 61 6C 2D
10: 6E 65 74 22
[BLEUART] RX:
data: 74 2d 73 65 72 76 65 72 20 6e 65 75 72 61 6c 2d 6e 65 74 22
[BLE ] BLE_GATTS_EVT_WRITE : Conn Handle = 0
[GATTS ] attr's value, uuid = 0x0002
00: 6C 2D 74 69 6D 65 20 65 2D 6D 61 72 6B 65 74 73
10: 22 20 7D 20
[BLEUART] RX:
data: 6c 2d 74 69 6d 65 20 65 2d 6d 61 72 6b 65 74 73 22 20 7d 20
[BLE ] BLE_GATTS_EVT_WRITE : Conn Handle = 0
[GATTS ] attr's value, uuid = 0x0002
00: 7D 0A
[BLEUART] RX:
data: 7d 0a
{ "id": 1, "name": "Leanne Graham", "use{ "lat": "-37.3159",na", "catchPhrase": t-server neural-net"l-time e-markets" } }
[BLE ] BLE_GATTS_EVT_WRITE : Conn Handle = 0
[GATTS ] attr's cccd
00: 01 00
[BLE ] BLE_GATTS_EVT_WRITE : Conn Handle = 0
[GATTS ] attr's cccd
00: 00 00
[BLE ] BLE_GATTS_EVT_WRITE : Conn Handle = 0
[GATTS ] attr's cccd
00: 00 00
[BLE ] BLE_GAP_EVT_DISCONNECTED : Conn Handle = 0
[GAP ] Disconnect Reason 0x13

Disconnected
[BLE ] BLE_GAP_EVT_TIMEOUT : Conn Handle = 65535
```

Start with the bleuart sketch and the BLEUART RX entries in the supplied serial log, comparing the 20-byte writes with the assembled output for the longer JSON payload. Reproduce the issue with the listed Windows, Arduino IDE, Android Bluefruit LE App, and firmware setup; done means the complete long string is received without dropped or reordered data.

Written by the indexing model from the issue text.