fix: remove C-ECHO messages

[feliwir]

Fixes #64

This can be validated by performing a STOW-RS against a DCMTK storescp:
Before alternating:

D: ===================== INCOMING DIMSE MESSAGE ====================
D: Message Type                  : C-ECHO RQ
D: Presentation Context ID       : 1
D: Message ID                    : 1594
D: Data Set                      : none
D: ======================= END DIMSE MESSAGE =======================
D: DcmDataset::read() TransferSyntax="Little Endian Implicit"
I: Received Store Request
D: ===================== INCOMING DIMSE MESSAGE ====================
D: Message Type                  : C-STORE RQ
D: Presentation Context ID       : 1
D: Message ID                    : 1595
D: Affected SOP Class UID        : CTImageStorage
D: Affected SOP Instance UID     : 1.3.12.2.1107.5.1.3.24338.4.0.260602921291147
D: Data Set                      : present
D: Priority                      : medium
D: ======================= END DIMSE MESSAGE =======================
....

with this patch applied only C-STORE:

D: ===================== INCOMING DIMSE MESSAGE ====================
D: Message Type                  : C-STORE RQ
D: Presentation Context ID       : 1
D: Message ID                    : 797
D: Affected SOP Class UID        : CTImageStorage
D: Affected SOP Instance UID     : 1.3.12.2.1107.5.1.3.24338.4.0.260604289371126
D: Data Set                      : present
D: Priority                      : medium
D: ======================= END DIMSE MESSAGE =======================
D: DcmDataset::read() TransferSyntax="Little Endian Implicit"
D: DcmItem::checkAndUpdateVR() setting undefined VR of PixelData (7fe0,0010) to 'OW'
I: storing DICOM file: ./CT.1.3.12.2.1107.5.1.3.24338.4.0.260604289371126
W: DICOM file already exists, overwriting: ./CT.1.3.12.2.1107.5.1.3.24338.4.0.260604289371126
D: DcmFileFormat::checkMetaHeaderValue() Version of MetaHeader is ok: 0x0001
D: DcmFileFormat::checkMetaHeaderValue() use SOPClassUID [1.2.840.10008.5.1.4.1.1.2] from Dataset
D: DcmFileFormat::checkMetaHeaderValue() use SOPInstanceUID [1.2.826.0.1.3680043.2.473.2.3472463830.1692.1087912310.331] from Dataset
D: DcmFileFormat::checkMetaHeaderValue() use new TransferSyntaxUID [Little Endian Implicit] on writing following Dataset
D: DcmFileFormat::validateMetaInfo() found 8 Elements in DcmMetaInfo 'metinf'
D: DcmDataset::read() TransferSyntax="Little Endian Implicit"
I: Received Store Request
D: ===================== INCOMING DIMSE MESSAGE ====================
D: Message Type                  : C-STORE RQ
D: Presentation Context ID       : 1
D: Message ID                    : 798
D: Affected SOP Class UID        : CTImageStorage
D: Affected SOP Instance UID     : 1.3.12.2.1107.5.1.3.24338.4.0.260602921291147
D: Data Set                      : present
D: Priority                      : medium
D: ======================= END DIMSE MESSAGE =======================
....

This improves throughput / performance for every case for me about 2x. As an additional benefit it also prevents termination of associations in some cases, giving about 20x speedups