CDC: ep_total_persisted is less than the ep_total_enqueued

Steps:

• Single node cluster
• Create magma bucket 

  Created bucket "NnC%qDVuZHQWccaRv1G4ocVC2YZ0ncQzBI7QORsJu_3lI2Y_e2hkii49AwM9Zqr_ZOQzY4Iap%2Qxq-51-556000" of type: couchbase

  [{num_replicas,0},

  {replica_index,false},

  {ram_quota,268435456},

  {durability_min_level,none},

  {storage_quota_percentage,50},

  {flush_enabled,false},

  {num_threads,3},

  {eviction_policy,value_only},

  {conflict_resolution_type,seqno},

  {drift_ahead_threshold_ms,5000},

  {drift_behind_threshold_ms,5000},

  {storage_mode,magma},

  {max_ttl,0},

  {compression_mode,passive},

  {magma_key_tree_data_blocksize,4096},

  {magma_seq_tree_data_blocksize,4096},

  {history_retention_seconds,20000},

  {history_retention_bytes,100},

  {history_retention_collection_default,true}]

• Create few collections with following history flags

  0x0:0x0:history:              true

  0x0:0x8:history:              true

  0x0:0x9:history:              false

  0x8:0xa:history:              false

  0x8:0xb:history:              true

• Load 1K docs into default collections and wait for ep_queue to drain

Observation:

Seeing ep_total_enqueued != ep_total_persisted

# /opt/couchbase/bin/cbstats localhost:11210 -u Administrator -p password -b "NnC%qDVuZHQWccaRv1G4ocVC2YZ0ncQzBI7QORsJu_3lI2Y_e2hkii49AwM9Zqr_ZOQzY4Iap%2Qxq-51-556000" dcp | grep remaining

 ep_dcp_items_remaining:              0

-bash-4.2# /opt/couchbase/bin/cbstats localhost:11210 -u Administrator -p password -b "NnC%qDVuZHQWccaRv1G4ocVC2YZ0ncQzBI7QORsJu_3lI2Y_e2hkii49AwM9Zqr_ZOQzY4Iap%2Qxq-51-556000" all | grep "ep_total_"

 ep_total_cache_size:                                       150890

 ep_total_deduplicated:                                     0

 ep_total_deduplicated_flusher:                             0

 ep_total_del_items:                                        0

 ep_total_enqueued:                                         27168

 ep_total_new_items:                                        1000

 ep_total_persisted:                                        26144