Improve logging in new SQL engine

[dai-chen]

Issue #, if available: #817

Description of changes:

1. Avoid dump request object which contains original query: Because we've logged sanitized query with request ID, actually there is no need to dump it again.
2. Add log for fall back reason: To make our development easier, added info log for fall back reason when explain queries. And also update README for user who wants to figure out this too.

Examples:

1. Logging for request handled by new SQL engine:

POST _opendistro/_sql
{
  "query": " SELECT age FROM accounts "
}
[aa74566f-ad79-4c3c-a221-5e1f6c6b88ed] Incoming request /_opendistro/_sql?pretty=true: ( SELECT identifier FROM table )
[aa74566f-ad79-4c3c-a221-5e1f6c6b88ed] Request is handled by new SQL query engine

2. Logging for explaining fallback due to unsupported syntax exception (ex. LIMIT) thrown by ANTLR parser:

POST _opendistro/_sql/_explain
{
  "query": " SELECT age FROM accounts LIMIT 1"
}
... Request is falling back to old SQL engine due to: Failed to parse query due to offending symbol [LIMIT] at: '
      SELECT age FROM accounts LIMIT' <--- HERE... More details: Expecting tokens in {<EOF>, ';'}

3. Logging for explaining fallback due to semantic exception (exception thrown manually from Analyzer):

POST _opendistro/_sql/_explain
{
  "query": " SELECT projects FROM employees "
}
... Request is falling back to old SQL engine due to: Identifier [projects] of type [ARRAY] is not supported yet

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

Merging #912 (ad66ca2) into develop (d170da3) will increase coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             develop     #912   +/-   ##
==========================================
  Coverage      99.85%   99.85%           
  Complexity      2149     2149           
==========================================
  Files            216      216           
  Lines           4851     4852    +1     
  Branches         323      323           
==========================================
+ Hits            4844     4845    +1     
  Misses             5        5           
  Partials           2        2           

Impacted Files	Coverage Δ	Complexity Δ
...ssion/operator/arthmetic/MathematicalFunction.java	100.00% <0.00%> (ø)	122.00% <0.00%> (ø%)

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