Bug 6692: clean up MonitorRequestBuilder

Bug 6692: clean up MonitorRequestBuilder

MonitorRequest can't be made immutable, so the utility of this builder
is to support fluent-style construction and addColumns() with multiple
columns. This patch removes unneeded methods from MonitorRequest,
drops the static builder() in favour of the constructor, adds support
for addColumns() with multiple strings, and constructs a new instance
for every call to build(). It also performs the skip cleanup in one
step instead of looping over the columns.

Change-Id: I89fd8a554551c3142efc82dbf6bb43417c6148ae
Signed-off-by: Stephen Kitt <skitt@redhat.com>