Purge requests can fail due to a gen_server call default timeout

[nickva]

Observed by a user in #4141 (comment)

 [error] 2022-08-01T16:36:25.158532Z [email protected] <0.7884.3264> fe8372ae91 rexi_server: from: [email protected](<0.6708.3264>) mfa: fabric_rpc:purge_docs/3 exit:{timeout,{gen_server,call,[<0.471.0>,{purge_docs,[{<<"37f60125fa241fd853c0457bd54160d2">>,<<"78108249-21ce-4981-b94b-ae07e33e9f9d">>,[{1,<<227,169,6,230,50,105,22,71,12,15,103,190,33,175,120,216>>}]}],[{user_ctx,{user_ctx,<<"*******">>,[<<"*******">>,<<"********">>,<<"******">>],<<"********">>}},{w,"1"}]}]}} [{gen_server,call,2,[{file,"gen_server.erl"},{line,204}]},{fabric_rpc,with_db,3,[{file,"src/fabric_rpc.erl"},{line,334}]},{rexi_server,init_p,3,[{file,"src/rexi_server.erl"},{line,140}]}]
[error] 2022-08-01T16:36:25.158755Z [email protected] <0.6708.3264> fe8372ae91 req_err(1108132862) case_clause : {error,[{error,internal_server_error}]}
    [<<"chttpd_db:db_req/2 L567">>,<<"chttpd:handle_req_after_auth/2 L320">>,<<"chttpd:process_request/1 L303">>,<<"chttpd:handle_request_int/1 L243">>,<<"mochiweb_http:headers/6 L128">>,<<"proc_lib:init_p_do_apply/3 L247">>]