Perf[MQB]: less allocations for ElectorEvent

[678098]

Problem

There are 56k allocations in 7.53 seconds on a busy broker (in ClusterOrchestrator)

:::::::::: :::::::::: ALLOCATORS >> Last snapshot was 7.53 s ago.
                                 | Bytes Allocated| -delta- | Max Bytes Allocated|  Allocations  | -delta-| Deallocations | -delta-
---------------------------------+----------------+---------+--------------------+---------------+--------+---------------+--------
        ClusterOrchestrator      |      54,164,000|         |          55,357,680|    856,637,235|  56,379|    856,265,300|  56,379
          *direct*               |      54,119,696|         |          55,313,376|    856,615,620|  56,379|    856,243,796|  56,379
          ClusterStateManager    |          38,688|         |           1,281,344|          6,175|        |          6,082|        
          ClusterStateLedger     |           5,616|         |             457,456|         15,440|        |         15,422|        

Changes

1. Provide d_allocators.get("Elector")) named allocator to mqbnet::Elector constructor (to have better granularity in stats)
2. void Elector::emitIOEvent: use a LocalSequentialAllocator to build a schema event
3. void ClusterOrchestrator::processElectorEvent: replace dispatcher()->execute(...) with ManagedCallback
4. bmqp_protocolutil: always set options first, decoder second; improve style

[bmq-oss-ci]

Build 3075 of commit a8e404c has completed with FAILURE