wrap indexer externalizers to write k/v length so index-server can process them transparently as byte[]

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(cherry picked from commit 122a06b3)

improve DSL: it is impossible now to forget to specify props as it is a first parameter to element creation, extract string constants where appropriate

(cherry picked from commit 8d106cf)

(cherry picked from commit 3eddd7a)

(cherry picked from commit 724ce61)