When maxHttpBufferSize is large (1e8 bytes), a payload of length 100MB
can be sent like so:

99999991:422222222222222222222222222222222222222222222...

This massive packet can cause OOM via building up many many
`ConsOneByteString` objects due to concatenation:
99999989 `ConsOneByteString`s and then converting the massive integer to
a `Number`.

The performance can be improved to avoid this by using `substring`
rather than building the string via concatenation.

Below I tried one payload of length 7e7 as the 1e8 payload took so
long to process that it timed out before running out of memory.

```
==== JS stack trace =========================================

    0: ExitFrame [pc: 0x13c5b79]
Security context: 0x152fe7b808d1 <JSObject>
    1: decodeString [0x2dd385fb5d1] [/node_modules/socket.io-parser/index.js:~276] [pc=0xf59746881be](this=0x175d34c42b69 <JSGlobal Object>,0x14eccff10fe1 <Very long string[69999990]>)
    2: add [0x31fc2693da29] [/node_modules/socket.io-parser/index.js:242] [bytecode=0xa7ed6554889 offset=11](this=0x0a2881be5069 <Decoder map = 0x3ceaa8bf48c9>,0x14eccff10fe1 <Very...

FATAL ERROR: Ineffective mark-compacts near heap limit Allocation failed - JavaScript heap out of memory
 1: 0xa09830 node::Abort() [node]
 2: 0xa09c55 node::OnFatalError(char const*, char const*) [node]
 3: 0xb7d71e v8::Utils::ReportOOMFailure(v8::internal::Isolate*, char const*, bool) [node]
 4: 0xb7da99 v8::internal::V8::FatalProcessOutOfMemory(v8::internal::Isolate*, char const*, bool) [node]
 5: 0xd2a1f5  [node]
 6: 0xd2a886 v8::internal::Heap::RecomputeLimits(v8::internal::GarbageCollector) [node]
 7: 0xd37105 v8::internal::Heap::PerformGarbageCollection(v8::internal::GarbageCollector, v8::GCCallbackFlags) [node]
 8: 0xd37fb5 v8::internal::Heap::CollectGarbage(v8::internal::AllocationSpace, v8::internal::GarbageCollectionReason, v8::GCCallbackFlags) [node]
 9: 0xd3965f v8::internal::Heap::HandleGCRequest() [node]
10: 0xce8395 v8::internal::StackGuard::HandleInterrupts() [node]
11: 0x1042cb6 v8::internal::Runtime_StackGuard(int, unsigned long*, v8::internal::Isolate*) [node]
12: 0x13c5b79  [node]
```

Backported from master: dcb942d