[3601] Addressed review comments, added ability to downgrade

[3601] Addressed review comments, added ability to downgrade

Several minor cleanup items based on review comments.  Implemented
support for downgrading files from newer schema versions:

doc/guide/admin.xml
    amended text on upgrading memfile to discuss downgrading

src/lib/dhcpsrv/dhcpsrv_messages.mes
src/lib/dhcpsrv/lease_file_loader.h
    revamped log messages to accomodate downgrading

src/lib/dhcpsrv/memfile_lease_mgr.cc
src/lib/dhcpsrv/memfile_lease_mgr.h
    added commentary to MemfileLeaseMgr ctor
    automatic conversion logic accomdates both upgrading and downgrading

src/lib/dhcpsrv/tests/csv_lease_file4_unittest.cc
src/lib/dhcpsrv/tests/csv_lease_file6_unittest.cc
    replaced tooManyHeaderColumns test with downGrade test

src/lib/util/csv_file.h
src/lib/util/csv_file.cc
    added CSVRow::trim()

src/lib/util/tests/csv_file_unittest.cc
    added CSVRow.trim test

src/lib/util/tests/versioned_csv_file_unittest.cc
    added VersionedCSVFileTest.currentSchemaTest test
    replaced tooManyHeaderColumns test with downGrading test
    revamped tests to check  getInputSchemaState() and needsConversion()

src/lib/util/versioned_csv_file.h
    Updated commentary to describe downgrade support

src/lib/util/versioned_csv_file.cc
    enum InputSchemaState
    input_schema_state_
    input_header_count_
    getInputHeaderCount()
    getInputSchemaState()
    needsConversion()

    next(CSVRow& row) - now supports downgrading rows
    validateHeder() -  now throws if called when no schema has been defined,
    and supports downgrading rows