Add built-in filter function for every Avro class

[kostja]

like()
regexp()

<
or
and

[Totktonada]

Plan:

• parsing of a filter (string or input object?)
• pcre
• eq, ne, lt, gt, le, ge
• and, or, not
• speedup using indexes

[Totktonada]

More from #43:

• IS NULL / IS NOT NULL

[Totktonada]

More from #39:

• [maybe] Support for nested filtering by connection_name.field_name.

[Totktonada]

Sink down to prio2 since PCRE support was extracted to #73.