introduce escape character for CIMI filters

[konstan]

The immediate need is in the queries of the free-schema data records.

Below is the extract from a data-record that was automatically ingested w/o any problem as there is no restriction on the content of the keys. The initial key was station/platform, which we namespace under the pre-defined data record key prefix (in this case gssc).

"gssc:station/platform" : {
    "last_date" : "2019-05-21T09:33:26Z",
    "agency" : "JPL",
    "site_name" : "Usuda / Japan",
    "IERS_DOMES_Number" : "21729S007",
    "status" : "OK",
    "last_rinex_file" : "USUD00JPN_R_20191400000_01D_30S_MO.rnx",
    "id" : "USUD",
    "count" : 141,
    "monument_description" : {
      "foundation" : "CONCRETE PAD ON ROOF",
      "height" : "0.15 m",
      "name" : "VIRTUAL"
    }
  },

Problem: ATM there is no way to query the attributes under gssc:station/platform e.g. with gssc:station/platform/id='USUD' CIMI filter. This is because the server splits on / and looks for platform under gssc:station, which doesn't exist.

Solution: introduce an escape character - e.g. \ - that would instruct the parser (for the keys only?) not to interpret the following one as part of the filter's ops chars.